OP - ignoring the arguments above, I think you should take control of this. While he might be in the middle of a crisis, that doesn't change the fact that he is being increadibly cruel to you and your DD. She has a daddy that's left, except he hasn't, and he's planning on coming back when it suits him, but might not, so you can't move on and start planning because you are letting him set the pace.
To me it sounds like he doesn't like facing up to the reality of the decisions he has made, he agreed to have DCs with you, now he might be regretting that, but he didn't say "definately not" and then you sabotaged the contraception 4 times, he has got you pregnant 4 times, it's only now that the second child is about to arrive that he's decided that actually he doesn't want this afterall.
So he didn't face up to what his decision to agree to DCs meant then (that he'd have to be a father to 2 people), and he does'nt seem to want to face up to what his decision to leave you means now, and you are letting him get away with not facing up to that.
Are you just DPs, not married? Then seek advice where you stand financially, start making this split official. Bag up any of his stuff that's left in your home and tell him to take it. Don't let him decide when he will be visiting your DD, set times that suit you and have him have his access time away from your home - he has a flat of his own, there is no reason that he should be coming in to have family time with you if he's left you.
It might not be that there's another woman, but he's creating a situation where it would be very easy for another woman to step into his life, whilst maintaining all the good bits of family life with you. Does his family and friends know he's left you at 8 months pregnant? If not, start telling people. The sooner this man has to face up to realities of his choices the better.