It is really great that you are realizing that this isn't a good relationship and that you need to end it. If you have to break up impersonally (like through email) because it won't work in person, then that's the way it has to be.
After that, you have to be the one to enforce the distance between you. If you have to change your phone number or block his number, then do it. If you have to ignore him at work or even start looking for another job, then do it. You have to make it a clean break with no back slide, otherwise he's going to get mixed-messages.
After a while, after you've given yourself a chance to heal and to figure out who you are and what you want out of a relationship, you can find a different guy who is all yours and you can feel good about that. No guilt, no fear, no sneaking around. Wouldn't that be so much better? Plus, if he'll cheat with you, he'll cheat on you. Would you ever really trust him, even if he were all yours? That's no way to live.
You deserve better than what you have now and he does too, he just may not know it. Be strong. It'll hurt like hell for a while and you're going to want to take it back, but you'll get through it and you'll find better in the future.