“Now he says he’s still unsure but wants to give things another go.”
Tell him to let you know when he’s absolutely “sure” and you’ll only consider it then.
You’ll never get the real reason he suddenly decided to do something this drastic, especially if somebody else was involved. Ever. He’s not making any kind of remorseful noises about commitment or being devastated to lose you, so even if you did get the reason he left, would you believe it or still have that gut feeling that somewhere, somehow, something isn’t quite right? It’s no way to live.
He won’t commit. He’s incredibly selfish and still keeping one foot in both doors, the marital door and the single man door. He needs to choose, now, and shut one door for good.
To think it’s ok to keep you in this horrible hoping/ waiting/ limbo/ anxious state whilst he makes his mind up is not loving, not about you or your life and marriage, it’s all about him.
He wants to fanny Ann about making sure his life is ok, his future is what he wants, he doesn’t get to lose out. He doesn’t get to use you as a place marker in his life whilst he does this. He appears to have no consideration for you whatsoever, dangling a future in front of you to keep you invested and prevent you moving on with your life whilst he sorts his out. Don’t allow him to do this.
Decide what YOU want OP, he’s done this, he dropped the bomb, the fallout is his problem. Don’t let it be yours, or allow him to think it’s yours. It sounds like you coped well with his leaving and made boundaries for yourself which he doesn’t like and now wants to have a foothold in.
You don’t need to ask how to rebuild trust, it’s redundant whilst he’s still saying he’s ‘unsure’. Who the hell can trust that? He’s trying to keep control of this. Take it back, the power is yours now. You’re already living without him, it appears you have something he didn’t value enough a while ago to stick with, but now he does. Maybe. Probably. Possibly. 🙄
For me he’d have to do a million miles better than “still unsure” before I’d discuss anything other than making the separation a permanent situation.
He’s not ready to commit yet, if ever, he’s said so himself in so many words. Leave him where he is OP, or you’ll have to endure him moving back in then out again for a second time. You’ve done really well so far, don’t give that strength and power away.