The world had changed in the time Stephen had been gone, though he'd expected that and even seen some of the changes in his multiple forays into the different timestreams. At that time he'd been speeding through them so quickly and simultaneously that none of the little things stood out to him. He saw riots, protests, wars, and, most importantly, whether or not Thanos was defeated. Thanos was all he'd really been focused on, trying to find a path that would save half the universe from being wiped out of existence. And he'd found it.
He'd found the way, set it in motion, watched as Tony lost the kid he'd practically adopted be turned to dust and then died himself. He didn't remember being dead. One moment he'd been staring at Tony and trying to commit his face to memory, the next he was staring at an empty, desolate planet with faint signs that a battle had occurred.
Then there was the battle and Stephen was going from place to place, gathering the rest of the sorcerers and portalling in all their allies to the main battleground. He'd seen this battle through the time stone, he knew what had to be done and what would happen but seeing a single event amongst a maelstrom of other events is very different to experiencing a single event in its singularity, he discovered as he fought against aliens and knew he couldn't save the one person he wished he could.
He heard the snap, everyone did, and it was like he snapped alongside Tony's fingers because that snap meant that Tony was gone, and Stephen hadn't even gotten to talk to him in between him dying and coming back and Tony dying.
Now Stephen heard the snap echoing through his brain whenever he closed his eyes. He couldn't sleep in his own bed because it wasn't just his bed anymore, hadn't been for six years even if he'd only gotten to enjoy one of those years. His back ached from sleeping on the couch but they'd gotten rid of the guest bedroom Stephen had kept due to having no other use for it and turned it into a workshop.
The singular bed taunted him, reminding him of all the moments he'd shared with Tony in or around it. Delivering coffee, hauling Tony away from work, waking up to Tony's sleeping face, falling asleep next to Tony. So many memories were held in that room and that bed, and that's all they would remain until they eventually faded. They were the last of their kind; there would be no new memories of Tony, no new memories of their life together, and certainly no new arguments over which mattress was more comfortable (Stephen had won that argument in the end and now he wished he hadn't).
Tony was gone and all Stephen had left were memories and a mattress.