Waking up 66 years into the future after willingly crashing the plane into the Arctic was a nightmare. Steve didn't want to live in a world without Bucky.
Getting him back made Steve the happiest he's even been in the 7 years since he'd woken up in a completely different world.
Losing him 2 years later made him miserable.
He got used to waking up next to someone, going out on dates, the late night conversations, being so in sync, having someone who understood him completely, even the relentless teasing (mostly from Tony, who was gone now as well). He got used to the domesticity and normality of it all, even if their lives were anything but normal.
All that was lost.
He couldn't sleep. Laying down on their bed hurt too much, so he slept on the floor, or the sofa, whenever he actually could.
He wore Bucky's clothes. They always smelled like him, and it provided some comfort.
He didn't eat. Natasha sometimes brought him food, completely silent, then come back the next day and find out it was untouched. She always gave him a disappointed look, but said nothing as she picked up the tray and sat down next to him on the sofa, or on the floor, or at the dinner table, or wherever she found him, and started gently feeding him. He never ate more than half of the food.
He didn't speak. He only nodded or shook him head, but never opened his mouth.
He felt pathetic. Everyone else who was left tried to help the outer world, Carol the other planets, and get them into order, while he was doing nothing other than cry on a daily basis and overall be entirely helpless. When Carol found Tony lost in the middle of the universe and brought him back along with a weird and scary blue girl, he didn't say anything. Not like he knew what to say, anyway. He looked at the brunet for 10 seconds tops, then retreated to his room and started sobbing, not going out or letting anyone in for the rest of the day.
This went on for months. With Sam gone, Nat was the only one who tried to get him to do anything, Tony retired with Pepper and everyone else always busy, and she was slowly succeeding. Most of Bucky's clothes stopped smelling like him and more like Steve, he slept a lot more, but never in their bed, he ate whatever Nat brought him, not everything on his plate and he sometimes had short conversations with her.
Things seemed to get better, and he thought, maybe he will be okay again. Not the same, but okay.
Until he found the ring hidden in Bucky's sock drawer.
Bucky wanted to propose to him. He wanted to marry Steve, wanted to spend the rest of his life with him.
Natasha found him trembling on the floor an hour later, huddled in his (Bucky's) emergency sweater, one that he swore to wear in only, well, emergencies, clutching the small box to his chest, silent tears falling as he whispered 'yes' over and over. Her heart broke. She knew about the ring. Of course she did. Bucky called both her and Sam and just threw the box at them, hyperventilating. Sam made a joke about "sorry, man, but i'm not into dudes," and they all laughed about it. The three of them spent a week forming a plan.
Bucky was supposed to propose the day Thanos happened.
Steve seemed unaware that Natasha was present, so she crouched and gently placed her hand on his shoulder. His whole body jerked at the touch, and he stopped shaking.
"Did you know?" he whispered.
"Yeah, me and Sam."
He opened the box and stared at the ring. He started shaking again, this time sobs coming along. Natasha hugged him tightly, rubbing his back as she started telling him about Bucky's plan, and breathed a laugh when she spoke about him panicking, thinking Steve would say no, her voice a soft whisper. By the time she was finished, there were no sobs anymore, just hitched breaths. She squeezed a little then let him go. Steve wiped tears from his face, looking up at Natasha.
She gave him a sad smile.
He looked down at the ring again and carefully took it out of the box, analysing it. She could feel his heart stop when he finally noticed the "til the end of the line" engraved on the gold metal. He slid the ring on his finger, then stared for a good minute at his hand, like something didn't belong. But it did.
"Thank you," he said, his gaze still on his hand, his voice barely a whisper.
Natasha gently raked her fingers through his hair, humming a soft russian lullaby.
Steve wore the ring for 3 years.
"He's not coming back and it's time I accept that, Nat," he said when he finally took it off and put it back into the box, placing it inside the drawer.
But he came back.
Scott came to the Avengers Compound 2 years later, explained the quantum realm, Tony created the time travel gps, Steve saw Peggy, feelings resurfaced, Nat died.
He cried. Natasha helped his through all those years, she became his best friend. His sister. He really loved her.
They got the stones, Bruce snapped his fingers, Thanos blew up the compound, they fought, everyone came back, Tony used the stones on Thanos and his army, he died.
Tony's funeral. Everyone was crying. Steve felt awful for Morgan, she didn't understand what was going on. For Harley, he never met him before the funeral, but the kid must've been very important to Tony. For Peter, the poor kid was so pure, and even if Tony would've never admitted it, he saw his as his own son, and Peter definitely saw his as a father. For Nebula, as well. Tony gave her lots of love, and her ice-cold cover was beginning to melt.
Steve still didn't get any time to talk to Bucky. He wanted to kiss him, to say yes a million times over, to spend the rest of eternity with him.
But he was the one putting the stones back, and he could save Bucky from all the years of suffering he went through. Join Project Rebirth sooner, stop Bucky and the rest of the troops from getting stuck behind enemy lines and get captured, search for Bucky after his fall in the Alps, live together for the rest of their lives. It was a simple plan.
He wanted to fix his mistakes, and this was his chance.
He wasn't going to miss it.