My boyfriend cheated on me with his baby mama? Should I believe him when he tells me that he won't do it again?

I found out that my boyfriend of 6 months has cheated on me with his baby mama. He says he's sorry and that he doesn't want to be with her & that he loves me and only wants to be with me. I want to believe him but the other day i saw a message she sent him, while he was in the shower, which read 'Okay thank you so much love ya' which I think that implies that they are still talking, frequently & he's just deleting their messages.

I feel that there really is not point for them to keep in contact unless it was something to do with their kid, which their kid lives with his mom full-time & the baby mama is a heroin junkie who's basically homeless, so i feel they shouldn't need to talk to each other hardly ever, the baby mama shouldn't even exist within our relationship.

But she is the one who is always contacting him first, asking him to do things for her (take her to work, asking him to buy her cigarettes, etc...) she'll send him provocative/sexy pics of herself (she knows he has a girlfriend that he is living with) The few times she has has their kid stay overnight with her, she seems to find a reason to involve him in it (asks him to take them to store to get ice cream, etc...)

When I bring up how i feel and what i think about the situation or how irrelevant she should be to him, especially since he keeps telling me he doesn't want to be in a relationship with her, he just can't seem to tell her 'no' on anything... 'no, stop sending me pics of yourself, we are not in a relationship anymore' 'no, i can't do that for you.' 'no, that would be disrespectful to my current relationship' 'no, I can't take you to go get ice cream with our daughter, I'll do that when she comes stays the night with me some other time.'

He makes me feel like i'm being a bitch for asking him to stop allowing her to disrespect us and our relationship. How am i supposed to trust that he would have sex with her again, if he can't stop hiding their interactions from me?

My boyfriend cheated on me with his baby mama? Should I believe him when he tells me that he won't do it again?
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