It sounds like there has been so much going on at the moment I can easily see how you would have been upset about the possibility of him cheating with this person.
I am very sorry for your losses....it is a hard hard thing to have to go through and men as try as they might, sometimes they just dont feel that pain as much as we do.
Can I suggest that maybe you guys go out on a date? Get your DD babysat, get dolled up, go out to a nice restaurant and just spend some quality time together. Even if its just cuddling up on the couch with a couple of movies and takeaway but it sounds like you both need to re-connect with each other again.
When I lost out 3rd baby....all I could focus on was getting pregnant again. It was all I could think about and it was an incredibly stressful time so I understand how you are feeling.
I am so glad he opened up....it must make you feel better in that regard and now you both know how each other feels you can both work on your relationsip.
I wish you all the best and tell this other biatch to f off!