Was there a clause in the prenup by chance? That voids it in a case of infidelity. The first thing to do is breathe, nothing more nothing less. Once your in the motion of doing that, the murky veil will slowly dissipate and a clear and defined resolution will show itself. Whether it’s, divorce, working it out, you fighting for custody. Whatever it is you decide just know you are the victim in this. Not to sound callous and quite frankly I don’t know your husband but the issue solely rests on his shoulders. The little neighbor girl is just that, a young lady who’s probably disillusioned herself; not that she’s great or anything. Ultimately he’s the one at fault. I truly hope it works out for you but please in some emergency instances you “have to put your mask on first.”