I'm in a very similar situation (even down to having met her through a support network, her being "straight" - well when it suits her, but in her eyes she is always honest and straight - prim and proper and judgmental, and us having very different values and little in common other than what brought us to the support group). I also don't know how best to deal with it. I have the added complication of still being a part of the online support-network (though she lives a good distance from me so I don't have to meet up with her).
I did manage to pull back a lot from contact, going from phone/text contact a few times a week to only speaking every few weeks or even every few months and only a small number of texts a month. I thought I could cope with this level of contact, but I find I am still giving it too much headspace and getting really annoyed by any phone conversations and often by her texts (though they are a lot less annoying as they are less time-consuming).
I just spoke to her tonight and feel really irritated by this thing that she does where she minimises mine and other people's problems. She engages in a sort-of suffering-one-upmanship, where she has an idealised view of other people's lives and an overly negative take on her own. Ironicallly she told me her mother used to drive her crazy with doing this to her.
Sometimes I feel like I am being gas-lighted by her take on my life, and even other people's lives. It is all part of this victimhood script thing she has going. I think this is the part of the relationship with her which is making me think I have to end it for my own sanity.
Funnily enough a few people that I know of have mentioned problems with her behaviour to her but she never takes it on board, is outraged at the idea of what they say, and then carries on with the same behaviour, resulting in the same outcomes over and over again. She falls out very regularly with people. I don't known how she manages it but she has a few rows a month with people (and then complains about people not maintaining contact with her!).
Anyway I am still thinking this through myself about how to best deal with it, as I don't want her to have a meltdown, for both our sakes. I started to challenge her more when I disagreed with her (which may have helped a bit both for me to feel I wasn't suppressing my feelings, but also for her possibly to pull back also as to her challenging her would probably make her feel "unsupported"). I think I will bring up the problem I had with something she said tonight and tell her how I felt about it and I might even say that I need some space as I find this sort of thing too hurtful and stressful.
I think this advice below was good (I had fallen into the trap of just not bothering to disagree with her when she made some comments, as initially I didn't realise I would be in touch with her for so long. Mind you you don't want spend too much energy arguing over every point either):
^Does she know how different you both are? I know I will nod and smile along with people saying the most rediculous things when I don't particularly care about their opinion. Have you been doing that whilst thinking she's a bit of a dick? Might she just think you are on her wavelength?
If that's the situation then I'd just start disagreeing when she says something you disagree with or talking about things you really like but that she doesn't. Not in a way that you're trying to be rude just in a way that you're talking like you would do with someone who you actually get on with without the politeness filter that you use for superficial meaningless chat.
If you do that she might work out for herself that you're not as suited to friendship as she thought you were.^