Honestly I just wish I understood better what prompts him to suddenly change tracks and walk back his comments, or to walk-back his walk-backs. Is it dueling advisors or something? Doesn't make much sense to me, beyond the general "I deny that I am subject to your rules." thing he has going.
I think it's a lack of advisors now. If you read people's accounts of him, Trump has always been impulsive in the sense that he throws everything he has at the immediate problem in front of him, finds a way to declare victory, and lets the little people handle the fallout and most of the actual work. He's not used to any kind of permanency, let alone accountability, and I think that's what you're seeing at play here: he's still trying bull-in-a-china-shop tactics and wondering why the shop's such a mess the next day.
He wanted to dispel rumors of Putin controlling him, so he goes and meets with him and tells everyone what to believe while Putin nods along. It doesn't work, and he looks weak and treasonous. Now all he wants is to look strong and pro-American, so he walks it back. But now people are talking about collusion, so there was no collusion and nothing to collude over, ever.
Hope Hicks was I think the one everyone else used to get him to be quiet, because she could make him think he was winning to matter what and didn't need to do anything. Without her there's no one to stop Trump oversteering on everything.
EDIT:
I feel like he's a guy who has spent his entire life removed from any normal social interaction who has no experience in what he's doing whatsoever. This means he has no idea what he's doing (ignorant) has no idea what non-super-rich people want to hear, and is suddenly under some very real scrutiny and therefore needs to be popular in a way he never even had to think about before in order to maintain his self-image.
Also this, plus he's never had to do anything but punch down before.