How do I cut off this friendship?

Honestly, thats a tough one because you can be pretty sure that no matter how you do it you're going to be the next "ex-best friend" she's bltching about.... In a situation like this, you just have to cut your losses and bail out. Be honest and, next time she wants to kick it, tell her you're not up for festival of complaints and take a p. She'll either get the point that she's too blthcy or she'll get the point that you're over listening to her... either way, you're free!