Judging by the context of what you're saying, I'm going to say that your relationship with him started out with him cheating on another girl. Your cousin.

If he was willing to break ties with her and possibly hurt her while getting with you, why would you be any different to him than your cousin was?

You said he already admitted cheating on you and slept with another girl a month ago.

Take a thorough look at what's going on here. He was with your cousin, and he cheated on her w/you. Now he's w/you. He's admitted he's cheated on you w/another girl who says she's dating him.

Hmm..

My advice is to let this person go as he's not going to change. At least, not because you want him to. He'll change when he is ready and only he can determine that.

Don't let your strong feelings for him be a factor in your decision as he's already willing to break the bond of trust and cheat on you. That was his decision. Let him live w/that.

Hope this helps you out in some way!