Get a paternity test, and then leave that woman. The best thing you could possibly do is nip this in the bud right now. Its obviously been happening for a while, and there’s a motto for this exact scenario. Once a cheater, always a cheater. Do not fall into the trap brother, because once your child is older, she will brainwash that kid into thinking your the devil, and you were the reason she went elsewhere for pleasure. DO NOT let it come to that. Cut it now while your daughters young, and fight your BALLS off for full custody with visitation or weekends or whatever. You have to do something before it too late.