My BF cheated on me & im struggling to get over it, it now affects sex

He cheated on me & we've had sex afterwards however of late I really struggle to relax during it, I don't feel good enough for him and I feel like I'm never satisfying him. In foreplay I do what to have sex with him but when it gets to penetration I clam up and it really hurts to make him fit. It's getting me down that he 1. Cheated and 2. That it's now having such a detrimental effect on our sex life i kno it's expected at some point but such a long time after doesn't make sense. All we seem to do is argue constantly which never makes it easy. I don't know what to do anymore