So my bf has a daughter with someone else for starters.... let say his BM name is Rebecca

He has a friend(let's call her Sara) that he knew before he even met me... they slept together. It's obvious to me that she still wants him. About two years later he had a daughter with Rebecca. Me and him start dating when his daughter is about 6 months old.

Fast forward to when his daughter is a little over a year old now. Sara goes to Rebecca and starts making up lies about me, saying I cheated on him, that I was pregnant with another mans baby then miscarried, that i was crazy, shouldn't be around his daughter, and that i blocked her on ig....

This causes Rebecca to tell my bf that she doesnt want their daughter around me. So we decided to have a talk me and Rebecca, Sara wasnt there. We sorted things out and everything is fine and dandy.

Fast forward to now, almost a year late and I found out my bf is still friends with Sara... now I'm not going to be the girl that tell him who he can and can't talk to, however I am going to tell him how I feel disrespected by her, dont trust her and feel like shes manipulative and that shes a liar....

Am I immature for doing so?

I'm over everything she did it was so long ago but I just wanted to tell him how I felt....

Does this make me childish?

Do I really just need to grow up?

When I expressed this to him I felt like he was making excuses like shes a good person and is good hearted and i just said lets agree to disagree. He said it didnt directly effect him so he has no issue with her but knows what she did was wrong.

He think I'm acting like a kid and I need to grow up... I truly dont understand