I WROTE HAPPY ENDING- it isn't very good, but HAPPY FLUFFY ENDING.

He'd expected to be slapped, or even for you to cry- you could be unpredictable with your reactions to things. He wouldn't have guessed you'd actually respond- not with coherent words, at least.
"Realized what?" Your voice was firm, cheeks stained with tears, and he watched you, astounded by the question, "Until what was gone?" The second one was sharper, more angry, and it hurt that you didn't assume it was you. Just another reminder of how little he made you think of yourself.
"I didn't realize that you were the most important thing in my life until I lost you... and when he did that-" Jim was getting angry just thinking about it.
"I wanted to get you back a long time ago, but I saw you both together and... he treated you well. Treated you right- like you deserved, and I didn't want to take that from you. But then I saw him go in that hotel with a random woman, and it pissed me off... you deserve more than that." He couldn't look at you, and wig every word his eyes sunk a little lower, a little more towards his hands. They fidgeted in front of him, and he was still waiting to be slapped. He would have to be stupid to think you'd give him another chance, after what he had done, but there was still a little bit of hope, but only a little bit. 
"I love you..." He whispered softly, eyes falling closed.
"N-No, you didn't- you don't!" Your words were already shaky, disbelieving of what he was claiming. Jim Moriarty did NOT love you- no way! But the sad look he said it with, the reservation of it... it made you question whether or not you were right or not...
"I did-! I do... I just didn't realize it until I lost you..." He listened to your footsteps as they came slightly closer, causing his eyes to open, being met with e/c. He would love to see you smile, but he didn't know if he would ever see the smile he'd idolized ever- not after he killed the man he watched make it happen. Still, his hand gingerly reached out to your cheek, cupping it in his hand before timidly pressing your lips together- slow and soft, just like he had watched. And to his shock, you kissed him back. To yours, he didn't try to push the kiss further, it didn't become heated or sloppy... it was just loving, because that was all he needed it to be. It wasn't long before he decided he liked this. Maybe even better than lustful kisses, and he could tell you agreed.
And when he pulled away, he saw that little twinkle, that little sparkle, that little light growing back in your eyes, and it made his heart skip a beat. He never wanted it to go out, again, and it never would, if he could help it.
"I love you, too, Jim..." The quiet revelation surprised him, but his face read hope.
"Give me another chance- I'll do anything, I swear it. I'll always have time for you, and if it would help, I'll fly out with you- take some time off work and come to your shoots... just, please... let me try again... let me be someone you deserve." His hands wrapped around yours, squeezing them in anticipation. His eyes looked desperate, lips parted in plead.
"Y-You would do all of that- change everything about yourself- for me?" He could tell you didn't believe it, but he would. You were enough, and he'd finally realized that without you there.
"I'd give up crime for you- I'd do anything, as long as I got to have you by my side." He meant it, every word. He'd prefer not to give up his second-favourite thing in the world, but he knew it'd be worth it, if he did. He watched you consider it, eyes filled with surprise, but your disbelief was fading.
"No more affairs..?" You said quietly.
"None." He said without hesitation. All he wanted was you, and if the last few months were anything to go by, he didn't think he'd have a problem staying loyal, this time around. After all, he'd been so busy thinking about you to even have a one night stand, despite countless opportunities.
"I-I won't be just a plaything...?" A pang went through his heart- a stabbing feeling that tore him apart.
"Not at all. I love you- hell, you don't even have to be 'mine'. I just want to be yours.." He admitted, hands venturing to rest on your elbows as you both scooted closer. You stared up at him in a silent awe, but nodded, agreeing to try again, he immediately kissed you again, a little more forceful, but not in a hungry way... more of a passionate way. A "you are the air I breathe" kind of kiss. You pulled away suddenly, and fear shot through him.
"I also want a guarantee that, if this works out, we won't just stay 'together' forever... that, maybe, one day-" He already knew what you were talking about, and a beaming smile broke out across his face, eyes welling with joy.
"Darling, I'll get down on one knee right now- I would love nothing more than for you to be my queen, forever." You grinned, squealing a small bit. That had been one thing he'd told you time and time again- "don't get your hopes up, we are never getting married" and things similar. Your arms swung around his neck, and, for the sake of being cliché, he spun you around. When he stopped, you had one more question.
"Kids?" You expected that to be crossing the line, but he just kissed your cheek firmly.
"We'll discuss it-" Your smile fell a bit, but he only chuckled, " I'm kidding. Whatever you want, my love. Anything and everything, anytime and anywhere. I swear it." You just hugged him excitedly, smiling like an idiot, neither of you having ever been happier.
"...I don't have to give up crime..?"
"No, Jim. No, you don't."
"Oh, thank god."