Why are you continuing to bother with her, in all honesty? Is it because she's friends with your mutual friends and you fear that if you stop talking to her your other friends will in turn stop speaking to you? Chances are, they most likely won't. This isn't secondary school.
We're grownups, OP. We have less time and energy for people who aren't TRULY our friends, OP, nor for those who don't invest a similar amount of energy in our friendships as we do, and most definitely not for those who continually let us down and don't have our backs when we need them to.
I'd view your friendship the same way you must view that quorn dish you (probably very pissed-off-edly) binned: something you've been forced to throw away because, despite the high amount of work you put into it, it's just not appreciated by the one it was supposed to benefit.
Sure, perhaps you may have to see her in future, because of your mutual friends, but if I were you I'd view her as an acquaintance now and be polite but distant when you are thrown together, not a friend. It really doesn't sound like she is one, anyhow.