Exactly this! Sam was a fundamentally different person once he returned to the world. And he had one person that he built his new reality on - Dean. Let this be stone number one...
Sam catches a lot of shit for not trying to save Dean at the end of season seven. Sam, who was having a hard enough time distinguishing between his warped memories and actual reality, watched as everyone simply vanished. From one moment to the next. Everyone that had ever meant anything to him was gone...
And those that he might have gone to for help in the wake of such catastrophic events? Bobby and Rufus were dead. Castiel had disappeared in the same moment as Dean. And Crowley? The bastard who at that point had only helped Sam when it served his greater good - he snatched up Kevin and departed.
The look on Sam's face as he realized that he was alone. Totally. Utterly and completely. Alone.
Season three Sam would have taken apart the entire world. Searched every single corner for some way to bring Dean back. Not stopping until the entire earth was a cratered out and burnt shell and he was the lone remaining inhabitant.
Season seven Sam? He was a PTSD stricken shell. He had nothing and no one. Until he hit a damn dog with the Impala. He poured every ounce of drive and determination he had into saving that animal. He couldn't fix anything else, but he could fix this.
Sam took on the trials as a form of penance, an apology for what he hadn't done. But even in that, he felt he failed.
Do you want to know what I confessed in there? What my greatest sin was? It was how many times I'd let you down. I can't do that again.
And Dean? His response to Sam in that church in the middle of the wreckage of their lives...
Don't you dare... think that there is anything, past or present, that I would put in front of you.
Which led to yet another traumatic experience for Sam. And yet another betrayal of his autonomy. Being tricked into accepting Gadreel, to be used by an angel to commit heinous acts. Sam is the one with the memories of killing Kevin. Long after Gadreel is dead, Sam is the one waking up in a cold sweat. Seeing his hand snuff out that innocent kid.
Sam was wounded and broken and on the cusp of possibly beginning to heal post-Chuck, and then as he held Dean in that godforsaken barn his own soul finally died. Hundreds, possibly thousands of years of torture had been unable to do what that act of losing Dean did to him.
His walking corpse roamed the earth for another thirty years, and he raised his child because Dean had asked him to, but he was dead inside. Just biding his time until he could be reunited with the only person who had ever completed him. Blurry wife? There was a reason his wedding ring vanished as soon as he got to heaven. As soon as he was reunited with his... everything.
Sam suffered countless traumas and violations. He grew and evolved as each new torment was piled on. Becoming quieter, making himself smaller, and just trying to fade from view. All the while, feeling as though he was failing everyone.