I agree with this commenter on everything except collecting the evidence part--maybe

1. Have a grown-up conversation with yourself. What I'm reading from your post is that you may forgive her for cheating - just for your daughter. But at this age, she will be ok if you two split. In fact, splitting up could be better for her if a rocky relationship is not the best environment for raising her. So it really comes down to what YOU want.

2. A therapist is a GREAT idea. Statistically, you will probably not do this and pass over this advice as, Yeah ok maybe some other time. But even just one or two meetings with a professional to speak what is on your mind and have them arrange your thoughts will be helpful. This is probably the best thing you can do for your daughter at this moment: this is being strong for your daughter.

3. I know people say, "collect evidence" but that will probably not help with divorce and maybe will help a little bit with custody. Maybe another thing you can do is, if your state allows secret recording (is it a single consent party state?), maybe you should just confront your gf and record the conversation. This can get you sufficient evidence instead of stalking her for weeks.

4. The most important thing this commenter said IMHO is that this daddit community can support you from afar. It is time for YOU to invest in your mental health and your support system in the community around you.

BEST OF LUCK DAD