Coping with your own betrayal

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Hi /adv/. I feel bad because I think I betrayed someone close to me.

Long story short; I had two female friends, one of which had a boyfriend that the other steal, presumably unknowingly. I was attracted to this one, but I was resolved we would only be friends, reason being she had no interest and I some kind of admire her for their intelligence.
I became her best friend, according to her own words. I had a really bad time a year ago, and I tried to have an appointment with her in multiple occasions for reason not fully understood by me; according to her, things at university hat got her depressed and/or some illness. There was always something. I met with the other girl by mere chance (I lost my virginity to her) and told horrible things about her; basically, that she was a slut, and was probably using me.
I did not wan to believe that. She (the one I was attracted to) trusted me a lot of things, and I trusted her in return.We used to share a lot of things but, of course, I was vexed by her tendency to never attend to see each other, just as friends to chat. We then had a lot of discussions... I no longer trusted her, specialy when she only accepted seeing me when she had an romantic problem, I felt that I was only for that, for validation when SHE needed it. We used to go to a comic con like event here on our city, and I refused to go with her last year (she was not exactly happy about that, in fact: stopped talking to me for quite a long time).
Recently we started talking again. She was very nice, in fact, bur he other girl showed up, and told me she was with he ex, again, who according to her was the worst human being ever. Well... a night came when I was very anxious about a lot of things, and really angry, and told her anything (they do not know each other "officially") and she told me I was a traitor and that she felt disgust of herself for trusting me.
Following in the comments...