Op I hope you know you did absolutely nothing wrong back when he turned up asking to get back together ...... He had repeatedly (it sounds like) ended the relationship, he'd done it - at least once - at a really bad time and in a shit way, he'd not shown commitment, and he'd broken your trust by messaging other women inappropriately (and it sounds like the only reason he wasn't cheating was lack of solid opportunity).
Your response to him turning up a while later saying he'd sorted himself out and could you now get back together was 100% normal/natural/reasonable/sensible.
No-one sane would have said "oh absolutely". He was lucky if anyone he'd acted that way towards was willing to have any contact with him at all.
So it is absolutely ridiculous/unreasonable of him to hold onto that and claim he hasn't got confidence in the future of the relationship because of it. He was lucky you entertained him at all. You have gotten back involved to some extent, and - surprise surprise - it's not you who's saying they're not sure about the relationship and can't commit, it's still him (!)
That vindicates your reluctance to get back with him straight away back then.
I think he's either a general commitment phobe/flake, or he's just not that into you. However he likes you as fall back girl, 'ol reliable and is perhaps scared of not having that option .... He's probably currently formulating his next strategy for keeping you in that position now that it looks like he can't play the "I would commit to you if you hadn't refused to get back together with me at first, I can't have confidence in the relationship due to that!" line for much longer.
(If it's true, he's being totally unreasonable. But it sounds like a line/excuse, even if it's a line he's convinced himself of).
I've not read Mr unavailable and the fall back girl but I'm wondering if it would be applicable to this situation.
I once had a poster describe my ex in a thread as "a piece of work" ... She was correct, and I think the term also applies to this guy. He is a piece of work.
Anyway, if he can't commit/doesn't want to.commit by now, it's clear he's had plenty of time and plenty of chances. He's still conveniently blaming you; for daring to not instantly jump straight back into a relationship with a guy who'd repeatedly dumped her and tried to cheat on her; he's an empathy free zone and seems to have huge trouble taking responsibility for his behaviour and realising that his behaviour has consequences.
(I wonder what his response would have been if the situation was reversed and you'd been the one repeatedly dumping him, messaging numerous other men in an inappropriate way, looking like you'd have cheated on him if the opportunity arose etc.... Do you think Mr special snowflake would have been welcoming you back with open arms if you turned up saying you were all sorted now?
He apparently can't even get past things when you have done nothing equivalent to him, (your massive crime being not instantly getting back together with him when he suggested it, which no sane person would have done) do if like to see him if you'd behaved remotely like he had to you.
I think that puts the relationship in perspective).