I really messed up. Please help me figure out what to do.

I have done possibly the most horrendous thing a friend can do and am really freaking out about how to handle it. I slept with the ex girlfriend of one of my best friends. They were together for about a year and have since been broken up a year, but it was a bad break up and my friend is still crazy about her. What makes it even worse is that I'm the one he comes to to talk about this stuff. The girl is also a great friend, and - it gets worse - we're actually moving into a shared house together with some other friends next month. She came down to stay for a weekend as there were some good nights on, but with alcohol and being alone in the room at night, one thing led to another. We had actually gotten together one night shortly before her and my friend started going out, so that chemistry was there but we just became good friends as their relationship. And ever since they broke up there's been a strict no-go mindset because he still hasn't begun to move on. I really don't know what was going through my mind when this happened. I just wasn't thinking. This girl is the loveliest person you could ever hope to meet, truely beautiful inside and out and if it wasn't for the friend situation then it would have been a dream come true. We were well into our drinks when she starting kissing me, and I should have stopped it then but I didn't, and now I feel like the most horrible person on the planet. I got caught up in it and now am in a nightmare situation with no idea what to do. And to make me hate myself that little bit more over it all, it felt like something that in another world would, could and should happen again, I've never experienced that level of connection before and as I said she's such an amazing person, but I know thinking that makes it even more terrible. I feel like I need to tell him, but the fallout is going to be immense, and I'm genuinely scared for our friendship. Throughout past breakups he's always been stuck on the girl for a long time afterwards, and never taken well to news of new involvements on the girl's part, up until someone new comes along for him. This is the longest he's been hung up on one girl though. I just want to say that this is so out of character for me I can't even begin to explain, I have never and would have never done anything to hurt any of my friends, I just don't have it in me to be or do anything that could cause someone pain or so I thought, and the fact that I now have is so bewildering to me that I'm lost and really in need of any advice, even though I don't deserve any help with what I've done. All I know is how sorry I am, and I can only hope he knows it too. And how it's going to affect the dynamic in the new house I can't even begin to guess but any outcome is going to be tense if nothing else. I messed up big time with this one, and I don't think there's anything I can do to make things right. I don't know what I was thinking. Please help?

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