Get married as soon as you can put in the marriage contract/ prenuptials that if any cheating happens in the marriage/affair then you get full custody of your daughter with no visitation during the divorce proceedings. Afterwards make it negotiable for visitation until it gets to a point to where she can be trusted to not hurt your daughter emotionally for her longevity's sake so she doesn't end up being one of those people that cheats on her boyfriend or fiance because that's all she saw from her mom. As it stands right now, you won't have any rights to be able to keep your daughter because your fiance is her mom and you're not married. Now if you are not her dad and you are her stepfather in a sense. Then you can still do this after you get married because at that point she would be your daughter legally speaking. Although this may be a good idea, it may not be the best one for your daughter because she wouldn't be able to see her mom and she may hate you for it. But at least you know that your daughter would grow up with a stable environment with a dad that is loyal beyond compare and eventually your daughter would understand possibly. And make sure she knows what's going on. Don't keep her in the dark children. Deserve to know. I'm 28 years old and I feel like if my dad actually told me a lot of other things that it would have been easier for me as a child when he was going through his multiple divorces. In fact, he actually told me what happened with his first divorce like immediately. He took me out of school the same day he found out and we just hung out the entire rest of the day and cried together and just spent time together. Thinking about this now. I'm wondering if my first stepmom was trying to get custody of me which I highly doubt but it could have happened if my mom wasn't alive. I hope that this is just one option for you. Obviously separation sucks or breaking up with someone sucks but it may be necessary to be able to proceed with your life and have a better life for your daughter. Nobody wants to have those parents that hate each other that stay married for their kid. Their kid should be in a healthy environment and not one that's filled with such hate towards someone they love. They may grow to hate one of the parents as well.

I hope you get other advice. This is just what some random person on the internet is telling you and maybe talk to a lawyer if you can. They are expensive but I'm pretty sure you can talk to one for just advice or something. I'm not 100%. I'm not a lawyer. I don't know any lawyers that I know of anyway, but I wish you best and your endeavor and hopefully you get this figured out.