How do I tell my boyfriend it's over?

So, I cheated on my boyfriend yesterday. I know that's bad but I really want to break up with him, I've wanted to for a while..and in my mind we are already over. The situation is that he was doing a ton of heroin and right now is in rehab for it. I have no way to talk to him til he comes back. I was going to break it off before he went but I felt if I broke up with him he wouldn't have gone at all, and he really needs the help(he was shooting up to 6 times a day). He was awful when he was using, not himself at all, and he knew how much it upset me, but it meant nothing. He's a really manipulative person, and he told me before he left "I will need you when I get out..without you my life would be pointless and all I will want is a needle in my arm." He has also told me in the past that our relationship is "perfect" except for his drug use and that if I think he's bad, I should see how other guys treat girls. They will treat me like sh*t. And I'll end up begging him to take me back..... Show More