No advice I'm afraid, but I thought I'd share as I can relate to your situation:
18 months ago I went through a horrific relationship breakup and hit rock bottom. I was glad to be out of the relationship, was but reeling from the appalling way my ex had treated me.
My best friend had always been the one I'd confided in throughout the relationship. She had a lot of issues herself and I went out of my way to help her - we were each others' rocks. I introduced her to my ex and they became friends through me - they worked in similar fields so had that in common, but she also saw his exasperating side and said she didn't know how I put up with him.
Two weeks before breaking up with my ex, I had a horrendous row with my friend and said some things I regretted (without going into too much detail, I had haywire pregnancy hormones at the time, ex was being abusive to me and in the middle of this she had a mental breakdown, decided to come and stay with us until she got better, was drinking too much and acting like a complete slag during this time and encouraged my ex to smoke again after he had finally managed to quit)
When I did leave my ex, miscarrying the baby, she didn't reach out to me, despite the fact I had apologised sincerely. Instead, she was a shoulder to cry on to my ex. Of course in his melodramatic way all she got to hear was his one-sided rant. I never asked her not to speak to him - all our other friends who saw what he'd done to me were smart enough to make their own choice to cut him off.
After a couple of months things calmed down enough for me to meet up with my friend, but the relationship wasn't the same - I no longer felt I could confide in her - how did I know it wouldn't get back to my ex?
What finally put the nail in the coffin was her deceit - she'd been going to see my ex in the new city he'd moved to but had hidden the Facebook photographs and updates from me so I wouldn't find out. I did find out however and when I asked her why she felt the need to lie to me she couldn't give me a straight answer (100% sure wasn't because they liked each other in THAT way)
I've now been with my new man for 16 months, am blissfully happy with him and the split with my ex is long behind me. Like you, however, I still struggle to come to terms with the break up of my friendship. She was the closest female friend I ever had, and it's left a void in my life.
I hate myself for looking at her Facebook profile all the time (we're no longer friends on there, but her profile has no security) I despise myself for resenting the fact she looks happy with a new man, that she has seemingly left the chaos of her old life behind. It's like I want her to suffer for what she did to me, yet ironically I am the one suffering caught up in this negativity. I was working on my laptop in a cafe the other day, and in she walked with her new man, completely ignoring me. I hadn't expected to see her as they'd moved away from my area, so it unnerved me and I ended up walking out, unable to concentrate.
Thing is, I don't even want to be friends with her again. By all accounts I'm not the first girl she's had an intense friendship with, followed by a cataclysmic fallout (in fact, I'm about the fifth) yet for some reason I can't let this pain and anger go.