I cheated on my boyfriend with my ex...?

I feel awful, and I'm not going to make it sound like I'm the victim. I know I was wrong, and what I did was horrible. Before my boyfriend and I started dating we talked for about a month I wanna say, we've only been going out for a little over a week. My ex boyfriend has constantly let himself come in and out of my life as he pleases, he's not the best influence on me either, he has gone to jail, been on probation multiple times, and is currently couch hopping. My ex and I would hang out before my boyfriend and I started dating, but once he found out we were going out he tried pulling the whole "I wanna be with you again card". I managed to ignore it and tell him we were only friends until last night.. My boyfriend is a great guy, and I don't want to lose him but I know I have to tell him. The only thing is that I'm afraid, his anger is awful, we got into a few fights already and it ended up with him punching walls and screaming. Cheating is wrong, it's happened to me numerous times and I never thought I'd be the culprit, but I really don't know what to do..