I have been with my man for 9 years and just recently found out that he may have been sleeping with his babymama the whole time. He swears it isn't true. But she says it is. I want to believe him. He says she just wants to break us up because he now has custody of there son so now he's in control when it comes to him. I would have never in a trillion years thought he would sleep with her. Plus she's a drug addict. Like really. Should I leave or should I stay? So emotional rite now.

He just got custody, of course she will lie, why are you even talking to her to start with.
It is fairly obvious that she is lying and you are falling for it. She will break you up and hope to get him back for custody. Or just to get even with him.

So you take the word of a ex, a drug user and someone with a score to settle over his word.

I am surprised he would not leave you for having such little faith in him