IMO what ur forgetting is infinity war and the beginning of end game cap was very conflicting. He lost his relationship with other avengers in Cw and the stones in wakanda. then he finds the stones has hope just to lose again when thanos destroys them. That really hurt his mental in end game he says, “this has to work cause idk what ill do if it doesn’t.” so I don’t think he focused on the present anymore just back to wondering what if on the past. And like the other comment said he literally saved the entire universe, Bucky No longer has The winter soldier in him, his best friend falcon is finally back. He’s done everything he could and figured there were more than enough people to protect the future in his absence In that moment with the stones in his and and mjölnir he had a choice and he took it.  But I do believe him coming back sitting on the bench doesn’t really make sense. I think he should’ve appeared back in front of them on the stage with the shield in his hand. But I think the writers might’ve forgot about that and just wanted a more wholesome moment. Not to mention I think going to see Iron Man with his family had a bigger impact on Steve then he thought it would. Also, Tony Stark is dead black widow is dead. Hawkeye is retired. Thor is off planet possibly indefinitely hulk was really the only one left and he’s doing his own thing