How do you get the courage to leave someone you’ve been with for 4+ years?

I’m afraid if I leave him I’ll never mind anything like him. He has more amazing qualities than bad but the bad involves a lot of mentally cheating, and some other past cheating (nothing physical).

The mentally cheating has nothing to do with love but more of sexual desire. Is sexual desire normal? I really don’t think he can help it almost. We’re both 21 and he’s told me what a huge pervert he was in middle school and HS.

I do believe he truly does love me, he wants a marriage, family etc. I couldn’t imagine a day without him, I would feel like I’m dying if we were no longer together.

What if I decided I wanted to leave him? How could I do it, I feel like I wouldn’t be able to. I wouldn’t be able to stand the fact of him being with someone else, I wouldn’t be able to take it..