Unless his ex-girlfriend had a gun to his head, it was his decision to have sex with her. He made that decision knowing that it would hurt you, and he did it repeatedly. The idea that he can’t help himself is not believable.
His cheating is not the only issue. According to you, he has untreated mental health problems. They could negatively impact your relationship, especially if he’s unwilling to seek treatment.
This does not seem like a relationship that’s in your best interest. He has cheated and that could be forgiven, but he is a repeat offender who has admitted that he has no self control. This would suggest that there is a strong probability that it will happen again and there’s nothing he can do to stop it. Thus, if you decide to stay with him, his ex will be in your life. Are you okay with that?
Only you can decide whether or not to stay in this relationship but you should choose wisely. Your choice will significantly impact your mental health and well-being. If you choose wisely, you will prosper. If you choose poorly, you will suffer. My advice is to seek counseling to assist you in this important and potentially life-altering decision. Please take care.
Dr. Kristina Randle