How do you end an affair?

For the past two months I've been having an affair with one of my longest, closest friends. He's been in a relationship for the past decade, prior to that we had a bit of a checkered past. And then all of the sudden, we started up again. He says he doesn't have a physical relationship with his girlfriend anymore, but he loves her and doesn't want to break up. I've known him to be with many other women while in this relationship, so I'm definitely not the first. He's kind of got it made -- the stability of a relationship with one woman and pion with the others. It's been great these past few weeks -- very fun and sweet -- but I'm suddenly starting to feel vulnerable, like he could say or do something that would hurt me. He'd been a jerk in the past, but he's apologized for that, and I'd like to believe he's grown up enough to not do that again. He's told me how much he cares about me and wants to preserve our friendship above all us. I agreed. Suddenly, I'm realizing that I don't trust him to take care of our friendship as much as I'd expect. I know the right thing to do is move on. So, how do you move on when you don't really have an opportunity to get closure? I know it sounds silly, but it bothers me to make up my mind about this while he might not really care one way or another or, even, notice if I just stopped contacting him. Sure, if he gets in touch with me, I can tell him it's time to end things and feel like a strong, empowered woman -- but it could be weeks before I hear from him again. I would look foolish/over-dramatic if I just wrote him and said "hey, it's been great, but it's over," right? I want to come out of this feeling awesome. Any advice?