Euthanasia: intentionally (legally, medically approved) ending your own life with proper equipment to stop pain and suffering.

This is usually only done if the patient will suffer forever and/or is soon to die.

Superman:

"I'll find a way, you don't have to do this." He kept insisting. He's Superman! He saves the world weekly, so why wouldn't he be able to save you?! He made you wait, claiming that he knows people.

He asked around. Every doctor he knew, every scientist, anyone who might be able to fix your situation. But no one could. It angered him more and more until he had to destroy an abandoned building to finally release that rage. But it wasn't anger. It was sadness. He can't give you up. But he has to.

So when he returned to you and saw you curled up in pain the guilt set in. He rushed to your side, putting his hand gently on your arm to rub and alert he's here. "I'm sorry." was all he could say. There was so much he wanted to, but it would all come down to sorry. So when you angrily told him, "I'm doing it!" he just nod. He could not even tell you not to raise your voice as you had every right. He made you wait, he made you suffer longer. So he simply said another, "I'm sorry."

As the euthanasia was being prepared he gave your cheek and every spot he could reach soft, tender kisses, whispering sweet words that made your frown turn to a smile. The last thing he said to you, and you to him, was "I love you".

Batman:

He knew it was coming, just a matter of when. The day you whispered, "I need it, please, the euthanasia. I can't anymore." He took your hand, gave it a kiss, and promised to give it to you. He did mean it, but he knew if he didn't ask the next thing he would regret it forever. "Can you hold on one more day?" "Why?" At your confusion he swallowed then went on one knee, fiddling in his pocket until he grasped the small box. Opening it revealed a ring you had been eyeing in a window store before all this began. "Will you marry me, tomorrow?" He had it all prepared, all that was missing was the couple.

You accepted. The wedding was held in the room. It didn't have to be grant, it simply had to be you, him, and friends and family. You both signed the papers, making it offical. He didn't mind if he was gonna become a widower the very same day he became a husband. At least he got to become your husband. All that mattered now was keeping that smile on your face until the end of the day.

When the day's end came, while both laid cuddled, he saw your pain was thrice what it normally is, for you had used up all your energy and tried to hide it for so long. He gave you a kiss, praising you for your efforts, calling you the best partner he has ever had. Your smile returned a little, "Thank you, I love you-" but the words began slurring. Eyes blinking, each time opening less so, and soon they stayed shut. Slowly he pulled the needle out of your arm.

"I love you, too."

Wonder Woman:

You are the strongest person she knows. So when you looked her straight in the eyes one day and said, "I'm ready to go" she knew you meant it. You hanged on as long as you could, now you are done.

But that does not mean she was ready for you to go. She shook her head, a whisper but then cleared her throat and loudly claimed: "You can't take the coward's way out!". The pained eyes you gave her made her snap right back to reality, make the cold shame wash all over her. Her eyes shot down, a hand to her mouth. That was the cruelest thing she ever said to you, and right before you would go. A mistake she will regret for life.

How could you be angry with her, though? For her grief was already in progress. It was obvious as the tears spilled over her hand and between her trembling fingers. When you held out your hand she grasped it, kneeling, her forehead to your knuckles, sobbing; "I didn't mean that! I'm sorry. Forgive me. Please! Please." If you did not she would not be able to ever move on.

Your hand slipped out of hers, causing her to clench air, a strained groan of misery as she ruined everything. But then a hand cupped her cheek and lifted her head. You looked like a magnificent deity looking down on its desperate worshipper. Your hand tugged, making her move along, until your lips connected.

"I forgive you. Now let me go."

Her eyes were unsteady but soon they steadied, hands clenched yours, and gave a nod.

J'onn J'onzz:

He had accepted he would have to give you up one day, but he always thought from old age or during a mission. He never thought it would go this way. He held your hand throughout it all, giving you the support and love you need during these times. When the euthenasia was brought up he knew you would take it, and he saw no fault in it. It's your life, your decision.

When you asked him if he's angry it made him realize his stern expression and silence must have come off wrong. He rubbed his thumb over your hand, assuring; "I'm not. I'm... trying to see what the future will be without you." He was without you every day, and now you'll be gone. There'll be a hole. First he had to give up his wife, his kids, and now you. He won't ever take anyone else, it seems this terrible faith for loved ones plagues him.

You suddenly tugged him closer, "Don't cry." He was confused but then realized he was. He was crying. "Don't think this is your fault, don't give up love." Hearing that only made more tears follow. It made him feel foolish that you have to comfort him now and not the other way around, so he assured you that he'll manage then put his focus on making sure you feel comfortable in your last few hours.

He did not find love again. Maybe it came across his path but he refused it. Any potential person he would look away, give the cold shoulder. He didn't mean it, that's simply how it went without him realizing until a Justice League member asked him why he does it. To which he answered, "My heart still belongs to them."

Flash:

"But you haven't finished this bucket list!" argued Flash as he held out a notebook filled with things to do before passing that he made for you that he was sure you would still want to try.

"I don't need i-" "Please!" "No-" Please." The last please touched you deep, for it wasn't begging or complaining anymore as he normally would at the thought of missing out on something but one that said this needs to be done so he can move on after you do. So you took it and checked off all you have already done. He watched closely, asking; "Really? Seriously? No way!" so you kept telling him the stories, and only halfway through all the stories did you realize this was his way to hear all of your stories one last time.

When the book came to its end he took it, "W-wait here! I'll write another list!" even though you are sure there are no more ideas he could come up with. So you held the book tight, telling a firm "Let it go" so he did, but slow and upset, knowing what is gonna follow. "You could have just asked the stories. Come." You held your arms out so he sat down then laid aside you, hugging you.

"I don't want you to go. I know that's selfish. That you wouldn't ask for it if it wasn't needed. But please... Please stay."

You would have. You would have given so much for that. But you can't. Realistically you can't. So with a kiss to his forehead you asked him to leave, for you knew he would not be able to stand by and watch them cut off your life support.

Hawkgirl:

She doesn't remember much of it. She recalls staring at a doctor as you and the doctor discussed euthanasia. Her head wiping to you when you called her name. The look in your eyes, which made her feel so terrified for there wasn't any uncertainty. You wanted this. You had decided on this the second it became an option. She didn't hear anything, not a single word, so it must have been so confusing for you when your question got answered by her hugging you and whispering, "I'll support whatever you decide."

The doctor left to get the supplies. As you two were left alone she was quiet despite having so much to say. Though... she wasn't sure what that would be. She wanted to tell you the most useless information, like how she watered the plants, had a plain lunch so plans to have a delicious dinner. Her focus was regained when you smiled at her and said, "It was nice while it lasted, wasn't it?"

She stared at you, her vision blurring as tears formed and slipped down her face. Her arms were around you again, holding you tighter than before, crying; "It was. Every moment with you was, even the bad ones. So long you were there it was nice and bearable. I won't ever meet anyone like you again."

There was a lot of crying the last moments, but with smiles while talking of all the nice moments you two have shared. Which only made it harder for her to watch you go but she doesn't regret it.
She did eat a delicious dinner later, though she did not taste it much as she only ate five bites before the utensils fell out her hands and her hands flew to cover her face as she bawled and sobbed like she never has before.

Green Lantern:

He thought it was fine, stable, so when you told him; "I want euthanasia" he just stared at you then began shaking his head, whispering "no", soon shouting; "No!". He refuses this. You can do this, you can heal, somehow. You will! Your hand tried to reach for his hand, but he moved away.

He stared as if you were someone else, or a new species entirely. You tried to argue that it's not his choice so he argued back; "It is, because you are my partner!" "What use is a partner who is in this constant pain?! This isn't a life I want to live! You are just being selfish!" And he couldn't argue back, because he was. He wants you to stay here for his own reasons at keeping you, not taking into account the misery staying even seconds longer is causing you.

Finally he took your hand, holding it tight. Hesitantly he asked: "Are you sure?". Your nod, small but firm, hurt him more but he knew it has to be then. Slowly he moved close, giving you a tender kiss, whispering, "I love you" so you whispered it back.

He can't believe it, he won't for months, didn't for years, even without you by his side he kept saying you are simply on vacation, that he will see you soon. Which, in a sense, was true.