do you want him to make the choice so that you don't have to? haven't you already decided?
if he's been saying and doing all the right things for the last few months, but you've been pushing him away, then did he take that to mean that you have already decided it is over, and perhaps that is why he has gone back to her?
do you want him back? it's like in your head you don't (because you could never trust him), but your heart isn't ready to let go. is that the case?
I think maybe you need to decide what you want for definite, and either give him the chance to come back and prove that he has changed, or actually go through with separating properly and formalising everything.
I'm not being harsh, know this is far from easy, but i worry that you are allowing yourself to live in limbo. you say you don't want him, but your actions say different. (exactly what I do!)
if I relate it back to me, I know full well that if H wanted to give things another try, I would let him come back and try (with a lot of grovelling from him first!).
But if he was willing, and I said no I've had enough, then I would have to accept it if he slept with other people.
Sadly, I don't even have that choice to make as H isn't even trying. So if you do want to give him a last chance, you need to let him come back and let him try, before it's too late.