I'm with you on this one. I'm on the verge of cutting off a close friend that I've had for 12 years now. I've noticed more and more unhealthy behaviors from her the last year and I feel we are outgrowing each other.
Where she has called me names, held things she's done for me over my head, and has used me as an emotional punching bag when things are collapsing in her life. Things I would never, ever do with her or any other friend of mine.
And, like you have written here, I think it's because I haven't placed those boundaries and stood up for myself. So, these friends continue pushing until I am burnt out and leave.
Now, I'm down to a few friends but they're not close. They're the type you have to make an appt to see once a month. It's going to be hard to adjust.