Alright buddy, maybe I have too much time on my hands here but I’d like to weigh in and I hope that it’s of some help to you.
For me, cheating really comes down to a lack of respect. If you love or loved someone in the past you should also respect them, I think those two emotions are mutually exclusive. I find it hard to believe that you could love someone who you don’t admire at least in some way or don’t respect enough enough to owe them some basic level of courtesy and to treat them properly as a human being with feelings.
If a relationship dies, if you no longer love the person, but you once did love them, you should respect them enough to say ‘this isn’t working, I’m sorry’ and move on whilst still preserving their dignity. As you can tell I have a pretty rigid set of beliefs here.
I’ve asked myself the question many times if I would stay in a relationship where I was cheated on (especially if there are children involved) and honestly I don’t know. Children deserve a stable home and access to both parents in order to be the very best versions of themselves and become self-actualised and accomplished people (or at least this helps).
I would suggest calming down! Take some time my guy. Time to really think things through. I would confront her, ask her why and see what she says.
Unfortunately like others have mentioned in the comments, the courts don’t really take something like cheating into account during proceedings pertaining to custody agreements. So anything you do now will likely have a big impact on the future for you and your kiddos.
Don’t be disheartened by this (I know it’s tough). Not everyone is like your fiancé but unfortunately dealing with people is like rolling the dice, you never really know what you’re going to get.
If you two can’t patch things up, just make sure your next partner not only shows you signs of love but also signs of respect. She values you and she values your sacrifices and efforts. Make sure it’s a give and take, that she would do the same for you as you do for her.
Best of luck.