Okay so Stan getting kicked out WAS NOT FORD'S FAULT and I'm tired of people blaming him for it. It was Filbrick's fault for actually being the one to kick Stanley out. Ford was only at fault for being a fucking teenager and listening to authority around him and internalizing that shit (kinda).
After Stan is kicked out, I think Filbrick spun it so that not only was it a good thing that Stan was gone, but that it was justified. The little he'd say about it was that Stan was never going to amount to anything and that he was just dragging Ford down, so thank God he's gone.
And though it hurt, I think Ford tried to internalize that (and mostly failed). He tried to tell himself it was justified because if it wasn't then what happened was horrible. And he did nothing to stop it. (He wouldn't have been able to stop it anyway.)
It's basically him wallowing in guilt and using "no, this was justified" as a really, reeeeally bad defense that he clings to for years and years afterward, to the point of him even saying so in the journal. He clings to it hard, and it's only after Stan's sacrifice in Weirdmageddon that he's able to finally go ".......no wait".
Like, without Filbrick (and later Bill) constantly telling him that it was justified, and a good thing, and don't worry about him he's FINE, I think Ford would have forgiven Stan on his own WAY earlier. It might have taken a few years, but it wouldn't have been over a decade.
And Bill never would have been able to tell Ford it was justified if Filbrick hadn't done that first.