Most Helpful Guy
You shouldn't be with someone who you don't trust. Cheating is survivable, but not in the context of further trust issues. So it probably makes sense to leave this person.
Try writing a letter to him, and not sending it right away. Just write it and look at what you have written, and the words that you need to tell him will be somewhere in there.
Please don't break up with people via text. The best way I find is with a phone call, because it personal but you still have a safe distance. Prepare for him to be hurt, but don't listen to a word he says about loving you for at least two weeks after telling him. Then contact him to say what's up, and then perhaps you can go on as friends, if you want to anyway.
Most Helpful Girl
Don't be nice, just be honest. You don't feel he is treating you well and you don't trust him. The end.