Can't seem to accept fiances daughter

My fiance and I have been together for 7+ years. He has not been faithful to say the least. When we met I had 3 kids from my previous marriage and he had one. 2 years in he got the mother of his son pregnant with son #2. I forgave him and accepted this child as my own, getting up at night to feed him, bathing, etc. We now have a 3 year old son together. The issue now is that when I was pregnant I found out that he had a daughter (he didn't know about her until she was more than a year old) but she is just a couple months younger than the son I accepted into my life as my own. He has no contact with his daughter or her mom. He only pays monthly child support. He has been stalking her FB page to get pictures of his daughter and he has told me many times that he wants to be part of her life. The issue is I can't accept her into my life. I already deal with the mom of his boys and I just cant deal with another. I feel really selfish and immature but it really hurts to think of having this child in my life. I know it is definitely not her fault and she didn't ask to be born but I have so much resentment towards her mom and my fiance. He has told me that once we get married he wants to legally pursue getting his daughter in his life. If I can't accept her should I just walk away from this relationship so that he can do what he feels necessary or should I just attempt to swallow my pride and support this in order to keep our family together? Please, any advice I can get.