My wife is leaving me and our two kids aren't mine?

Right now, I'm at the lowest point I can be at. I have been married 6 years and I was with my soon to be ex-wife 8 years. She has two children, girls, 2 and 3 weeks who I believed were my babies before 2 months ago. We had a great relationship or so I thought. I treated her extremely well but she says that so doesn't love me anymore. She started having a an affair 4 years ago with the man she's now leaving me for. I work out of town a lot and like a fool I thought I could trust her. 2 months ago, she informed me that she had cheating and that our 2 year old and the baby she was pregnant with are not mine. I felt like I wanted to die. How could she have with him in our house, then the next day smile in my face? Its just not human. She moved out after that and filed for divorce. I was not at all interested in staying with her but I wanted to know if these were my children. I tried calling her to see how our daughter was but he always picked up and said that its his daughter and to stop calling. I want to literally shoot this guy in the face, repeatedly. I didn't want to fight this guy so I never went over to their house. When the baby was born, her friend called me and I went to the hospital. They both got pissed and said I had no right to be there ruining "their day" and thats when I finally did fight the guy and ultimately I got escorted out the room. 2 weeks ago, we got a DNA test and she was right. Both the girls are biologically his. I'm so pissed off. Knowing that she's been deceiving me this whole time sucks enough and I now have found out that she was bringing the 2 year old around him before telling me about the affair confusing her since she was calling both of us daddy. I hate her so f***ing much. She called yesterday saying she wants to be with him and wants me to sign over my rights to the 2 year old. Since I thought she was mine, my name is on her birth certificate so legally I'm her father but its not fair. I love her, she's my baby and its not fair that I'm being punished. I still want to be a father to her. My family says I should just do it and get them out of my life but the pain would be unbearable. 2 years I've invested in this little girl gone. I'm so f***ing hurt and betrayed. What would be the right thing to do?