I messed up, need help please.

Hi, I'm 17 and I've been best friend with this guy since we're 8. He's kind of mean with his girlfriend (not violent) and they almost broke up 2 weeks ago. He didn't have a phone this summer so he used hers to talk to me, so she found my number and called me. She told me what happened and she said she wanted to hang out. I accepted only with good intentions, I thought it would make her forget for a while, and maybe she would give him another chance.

He didn't call her for a week, she wanted to see me more often. We met several times in public places and one night she asked to go at my place. I could see from her tone that something was wrong, still trying to save their relationship, I picked her up and brought her home. We went on the roof and it started to be cold outside, and we rolled ourselves in the only blanket we had. I dropped her off at her house around midnight, when I came back home, we texted a bit and we both confessed our love for each other. We couldn't let that happen, since I couldn't live with myself knowing I stole my best friend's girlfriend, and she didn't want to betray him either, so we decided we would be "siblings".

A couple of days later, she slept in my bed, me too (I was supposed to sleep on the couch, but we ended up sleeping together, as siblings, so nothing wrong happened). Then we spent the next day cuddling on the couch and watching disney movies. He called her around 9PM, they broke up and got back together over the phone (They were both yelling so I heard most of the conversation, he made her feel guilty for something that wasn't her fault at all and used that to make her stay with him). She told him "everything" -- basically he only knows we have already chilled together... He was mad at her, because he had already told her not to talk to me (I guess he knew it would happen, we're basically identical but I have less anger management issues), but he said he couldn't be mad at me more than five minutes after all those years.

I felt bad for what happened, I explained to him I didn't mean to fall in love with her, and that I would never have attempted anything. He said he understood and it was OK. Yesterday, I asked him if I could still see her from time to time. He said he'd rather not, so I said OK. She called me in tears like 5 minutes later saying they almost broke up and he said if he found out she still called/texted me or anything, he would break up. I called him and said she didn't have anything to do with it, she had already told me she couldn't see me any more so I called him to see if he could understand how I felt. I told him he had to be mad at me, not at her. He said "Whatever, I don't care" and hung up. He called her, they broke up and got back together again, probably the same way, and she told me we couldn't talk to each other again because she had suffered too much. Right now she's "on probation" : if he suspects anything it's over, and he'll make her feel bad about it.

I'm having a hard time not talking to her, but what's worse is that he doesn't answer the phone either. They are the only two people I like to spend time with, and I don't know what to do. I don't want to screw up their relationship, even if I'm pretty sure it would be for the best since they're constantly hurting each other, but I don't think I can live without at least one of them in my life... I need a way out ASAP, I can't sleep and my grades are already suffering. I'm supposed to be studying right now but I can't concentrate at all. I know I screwed up bad, but he doesn't even know that. Is there a way this could end well?