OP, I know how you feel, I had a very similar situation with an extremely close friend of many years.
I loved her to bits and we got on so well, we'd been through a lot over the years with our lives and always, always were there for each other and had each other's back. Unfortunately she became jealous of something that I was excelling at, because she wanted to excel in it too, which is fair enough. However she started acting in quite a nasty way and doing things to sabotage my success, I had suspicions she was doing this but then it was confirmed to me by a couple of other people.
I know it sounds stupid, but I really didn't want to confront her about it, I hate confrontation and didn't want to ruin the friendship, so instead, I posted anonymously on a forum (not MN) venting, basically asking whether I was being unreasonable for feeling hurt and pissed off. Everyone that replied to my thread said god no, what she was doing wasn't on, have it out with her etc etc.
I tried to talk to her about it but she lied. So stupidly I just left it, time went on and our friendship got better again for a while. Until she started becoming jealous again and the one night, OVER A YEAR since I posted that thread, I got a phone call from her saying how hurt she was, she's just found a thread that I wrote about her blah blah and that she was so disappointed, she needed to end the friendship.
I couldn't even remember the thread when she first mentioned it as it had been so long since I wrote it! Then I remembered it, I was honest and said yes, that was me, I was sorry but I wasn't bitching, I was simply stating what she had done and asking posters whether I was being unreasonable for feeling hurt and annoyed. She then went on to say that she hadn't done what I said she'd done. I said, yes she had, 3 different people had come to me and told me and why would they lie?!
She then relented and said 'well alright then I did, but only the once' it wasn't the once but I just couldn't be arsed to argue. I asked how she'd even found the thread as she didn't frequent that forum and had no idea of my username (my username was pretty similar to this one, so she'd never have known to search it!) she said a friend had alerted her to it. I said BS, how on earth would a 'friend'
Just stumble across it as it had been written a year earlier and was just lost in cyberspace, I tried to find the thread and it took me 20 mins and I know my username!! She kept on with that story for ages and then a couple of days later she said 'well actually okay the friend saw it a year ago when it was originally posted but just told me the other night' I said well why would they do that?! Someone doesn't just think 'oh I know, X & Y are good friends, I did see that thread a year ago that X wrote about Y, I'll go and tell her now' BS, she must have been bitching about me to this 'friend'
For the record, my post was completely anonymous, no names, places etc were mentioned, apparently this 'friend' just recognised the scenario, which says a lot!
Anyway, that was 2 years ago now and we haven't spoken since. I miss her terribly every day, I literally did lose one of my best friends.
However I'm angry because I only stated the truth in that thread and only posted to vent anonymously rather than confront her as I knew she'd just lie about it. However, she really has gone around painting me to be a massive villain. She's told anyone who will listen how I 'bitched' about her on an Internet forum, yet fails to mention WHY I was bitching on a forum in the first place. It really pisses me off to this day that I still get some people giving me dirty looks etc when I know they've only heard one side of the story.
It doesn't make me miss my old friend any less though
I also have said to people, and people have said to me that if it was the other way around, I wouldn't have ended the friendship over it. At the end of the day, if I was being a prick to my friend and doing something to sabotage their success at something and I saw they'd found out and expressed their hurt on an Internet forum, I would accept that that's what I deserved really doe doing it in the first place. I may have been a bit hurt and cut them off for a few weeks, but I certainly wouldn't have cut a long term friend out of my life completely for that. It's the 21st century, forums are used for venting all the time.