Wow. So, a lot to unpack here. First, let me say that I am also a 23-year-old gay Catholic, so I can at least relate to you on that side of the issue. As far as that goes, I'm still figuring it out myself, but one good starting point I have is to suggest reading some of Chris Damian's blog posts where he does a pretty good job of writing about the various issues involved for people in this situation.

Now, for the bigger issue. My personal advice would be to sit down for a long talk with your fiance and tell her everything. It'll be very hard, but it is really for the best. If you were to go through with the Catholic wedding, you'd have to realize that there is no going back - you're locked in for life. You should still try to be a part of your child's life, but do enter into a marriage under these circumstances would be unfair to her, the child, and you.

Finally, my last piece of advice would be to talk to someone you can trust and rely on. You're going to need a lot of help to get through this, and having someone there with you will be huge. If you don't know if anyone, I'd suggest asking a priest for some private time to talk about the issue. They can be surprised understanding, and will be able to guide you on next steps, especially if you really do believe and want to do the right thing in this context. An added bonus, if you're feeling too nervous to talk this over at length right away, you could make an easier first step by going to confession, where you know that anything you say will never leave the room unless/until you decide to bring it up outside.

I realize this will be a difficult journey for you, and I will keep you in my thoughts and prayers as you work your way through dealing with it. Please feel free to reach out to me or anyone else if you ever need someone to talk with about it.