He was calmed down from his meltdown and assured that he wasn't a monster for feeling negative emotions about his life, he was allowed to have them and express them.
But his PTSD was NOT cured. When he was crying after turning back, that was still PTSD related. He was finally letting it out.
Then even when we had a short time jump to a few months ahead, he mentioned that he was STILL seeing a therapist.
Pretty much that ^^, plus the overarching problem not being related to a lack of affection from others -everyones love for Steven is pretty clear - but rather Steven being unwilling to reach out and communicate his needs and frustrations. Steven was unable to cope with the idea of needing help rather than providing it, and overall desperately needed to carve out a new role for himself now that the universe didn't literally rely on him. Hence why he leaves at the end - he's following his dads advice to seek inspiration for a new start, but now with the understanding that everyone 100% has his back, whatever happens. Its the healthy version of Gregs relationship with his terrible parents.
(Side note: The episode about Gregs history provoked an interesting discussion about parenting between me and my wife, but why anyone would point to it as some atrocity on Gregs part is totally lost on me. Yes, growing up in a stable environment is good for a kid, but Gregs parents were shits, and the only reason Steven didn't see that was because he was an angry teen in a terrible headspace. Seriously, they forced Greg to be a MATHLETE and a WRESTLER. GREG. Plus, you know, completely ghosting him)