The Importance of Being a Dad

He's 28 who was with his ex for eight years and they have three kids one of them is his own only. She had a kid in past relationship and then three months into getting into the relationship with her he got she got pregnant. He chose to take the baby on and raise the child as his own. However, this is where it gets a bit complicated The child's biological father is now back on the scene after she promised she wouldn't let him be a part of the child's life Now we aren't together anymore. I feel like she's going to cut me out. What do I do?