Have had to do this a few times, I am woeful for attracting 'lame ducks' as my mother would call them, both friends and partners.
First friend was very jealous and bitter about any little success I had, would make snide little comments about me and at the same time panic and overreact to everything and rely on me to calm her down. I got sick of it finally and told her I wanted nothing more to do with her.
Second friend, I suspect had histrionic personality disorder, based on her behavior and her reactions to things. Would tone it down a lot around me but let it fly full force at other people, and I should really have been suspicious that she drops (or is dropped by) her entire social circle every few years. She intentionally seeks out vulnerable people who are more likely to keep quiet about the things she does. I dumped her when she stalked me online and tried to corner me about posts I had made that weren't even about her. I'm talking seriously creepy stalking, she read through a back log of about 200 pages to find what she wanted to find, and the only way she could have found my profile in the first place was by looking at my phone when I was out of the room and making a note of it.
Third was actually my best friend of about 10 years, and I am still a bit gutted over that, but I wasted those 10 years and got nothing in return. She could be a good friend but she was utterly self-absorbed and caught up in severe depression that makes her very resentful. I had the worst year of my life last year and I could just tell when I was trying to talk to her that she was waiting for me to shut up so we could talk about her problems, though we had been talking about her problems for hours already. In the past I had supported her financially when she needed it, encouraged and found jobs for her, helped her with college work, when we lived together and she expressed that she didn't like where we were living I did all the legwork when it came to finding somewhere new to live. Nothing's changed, she lives with her partner in her preferred country, she has a good job with a lot of personal freedom, she has a good therapist and her partner is very supportive and she's still miserable. In the end I had to let her go for my own sanity.
I know this makes me sound like I get rid of friends a lot, but honestly I have a lot of good friends who are there for me and I'm there for them when they need it. A really good friend can make you realize how much crap you've been putting up with.