Desperate to find a solution for your declining health, Gaster had moved some of his lab equipment and other things into a room of his house, constantly conducting tests and monitoring your vitals as well as your soul.
With the information and the previous discoveries with you and your ability (even if it was just once) to pass through the barrier, Gaster constructed new plans and theories. He genuinely believed that whatever you were missing to fully heal you was somewhere on the surface and because of that he wanted to find a way to break the barrier once and for all.

Asgore had also somewhat recovered from his depressed state with Gaster picking up the pieces. Asgore tolerated having you down here in the Underground since you meant so much to Gaster but other humans would not be lucky enough to be shown mercy. The fear monsters once had towards humans had faded and was replaced with anger. Humans killed their prince as he tried to fulfill his best friend's dying wish.

There was no honor in what they did and they vowed to show just as little sympathy for any falling humans as they did with their prince.

With Asgore knowing about the capabilities of a human soul, specifically knowing about the fact that you managed to pass through the barrier, he asked Gaster to take any information and data he has about your soul and find a way to contain your magic or soul power to eventually use it to break the barrier and reclaim what is theirs.

Gaster protested about this for a long time. He did not like constructing things that could potentially harm you but as royal scientist, he had the obligation to provide Asgore with whatever he asked for. While researching your condition, he further studied the properties of your soul, to prototype the very first container that should be able to contain a human soul.

Something inside of him hoped that this would be the solution to finally break the barrier, but he also hated that it could as well be used on you...the one he loved with all his heart and soul. The power he held in his hands with this container terrified him, but he knew it had to be done. He procrastinated developing the device for too long and Asgore was growing impatient.

Gaster was scared that if he ended up not fulfilling his wishes, it might put you at risk and in the worst case, Asgore would change his mind and no longer grant you immunity to the 'kill all humans' order.

You were laying in bed as Gaster was away, working on the prototype of the container. He was more distant, the weight of the situation weighing heavily on his shoulders. You didn't blame him. He was torn between one and the other and he was desperately trying to keep everyone over water, while trying not to drown himself.

It was too much.

Whenever he got home, he looked worn out and tired. He barely talked, his face always buried in notes or blueprints.

He always holds his breath when he checks your vitals, then smiles at you without saying a single word and gives you a reassuring kiss on the forehead.
You knew it was meant to encourage you, but he stopped reading the analysis results to you which was more than enough for you to know that things weren't getting better.

Sometimes his eyes started glowing in different emotions. He tried his best to suppress it to not concern you but his eyes betrayed him and you occassionally caught a glance of the color. 

Your skin grew more pale, you suffered from immense weight loss because you were struggling to keep whatever you ate and drank inside.

It seemed like whatever was going on with you was spreading faster with every day and there was no way to stop it.

You wake up from your light slumber as you hear the door open and carefully sit up as Gaster enters the room. Every muscle in your body hurt from that slight movement but you tried to smile at him nevertheless.

'Did I wake you, my love?', he signed and approached the bed. You shake your head.

"No, it's fine. I couldn't really sleep anyway. How have you been? Made any progress?"

Gaster sighed and leaned down, giving you a soft kiss on your head before turning to the screen and checking your vitals.

As he leaned down, his scent hit your nostrils mixed with a hint of smoke. Smoking was something he had started only recently.
He claimed that it eases his mind, calms his thoughts and makes him relax.
Some days or nights the smell was worse than others.

'I think I am close enough. At least this time the prototype didn't short-circuit.', he chuckled weakly before turning his attention back to the screens.
His floating hands picked up the plate of food from your nightstand as well as my empty glass. The food was barely touched.

'I am going to get you some more water. Lay back down and relax.'
He disappeared into the kitchen, and you lean back with a small sigh.

You glance around the bedroom, your eyes landing on the stack of books that are on his nightstand. All of them are medical books about various diseases as well as cancer.
Everything and anything that could maybe explain your condition.

It was more than heartbreaking for you to watch Gaster try so hard without finding an actual solution. He was tearing himself apart and you couldn't really do anything to ease his mind.

As Gaster returns with a glass of water, he sets it down on your nightstand before sitting down on his side of the bed, just pondering.
His gaze was distant and focused.

You slowly reach out towards him and lightly touch his hand. It was enough to snap him out of his thoughts and he turned to you, giving you a soft smile.

After a while, he stood back up and got ready for bed, laying down next to you. He immediately wrapped his arms around your waist and nuzzled his face into the crook of your neck. His floating hands glowed softly in blue as they manifested and floated into your vision. 

'You are my everything, [Y/N]. Never. Never forget that.' 

You feel him slightly tremble next to you as he buries his face deeper into your neck. He was desperately trying to keep it together, but you knew he was about to cry.

Your heart ached as you heard his strained breathing.

He pulled you closer, tightening his grip around your waist as if you would disappear the moment he would let go.

The most terrifying thing about it, however, is that this could be the case.

You were blessed with immortality, but you knew that someday your luck would run out and now the outcome was not so certain anymore. 

'I am trying my best. I really am but I am worried my best just isn't enough anymore. I don't want to lose you. I can't bear the thought of living my life without you. I am scared of failing. Of failing you. I am so scared, [Y/N].' 

He confesses and you feel his tears staining the skin of your neck. He was sobbing softly, his whole body shaking and his bones rattling slightly. It hurt so incredibly much to see him suffer like this. It made you tear up yourself and you used all your strength to turn to him and wrap your arms around him as well.

"I know, G... I know you are trying your best. I see how much you do for me every single day and I am so incredibly grateful. You make all of this so much more bearable."

You try to reassure him, but it was getting progressively more difficult as the sliver of hope continually grew thinner.

You hated to admit it but sometimes you caught yourself thinking if it was better if you just died.

Maybe it would help him move on faster and stop his suffering.

You didn't want him to feel guilty or not good enough for not being able to 'fix you'.

You didn't want to plague his mind every day and night with him fearing for the moment where you fall asleep and never wake up again.

You just wanted it to be over.

For him to be happy.

"I am scared too, G... But you give me strength. You make me not lose hope. I want to be with you. Forever and always. And I will not stop fighting."

Gaster cried harder, clinging to your shirt desperately.

'I love you so so much...please don't leave me..'

Your heart feels heavy as you read his words, tears streaming down uncontrollably.

You really didn't want to leave him, but you knew by now that this fate was inevitable.

You thought you'd have forever.

You thought you would never see the day where you were worried about suddenly not waking up again.

I am so sorry, G...