She was abusive, and I don't think she really realized it.
We had both come from abusive backgrounds but had largely escaped it. We bonded pretty quickly and got along really well, but she had this deep seated belief that she was damaged that led hear to kinda treat me like shit most of the time. The single biggest thing was that she simply could not tell the truth. As a point of pride, she would always say "Nothing I say is a lie" or "Everything I say is the truth", but that's not quite right: she always say things that were true, but not necessarily the truth. And she was really good at using it as a way to mislead. Most of this I was somewhat understanding of - I have trust issues too, so I kinda figured it wasn't intentional or intended to be harmful.
That was my first mistake: I started making excuses for her, and defending her actions for her.
Anyways. At some point the whole thing started bugging me. We were inconsistent friends at best - she'd go from largely ignoring me, to having in-depth, engaging conversations like we were best friends when we ran into each other. It seemed like she was keeping me compartmentalized in her life, which was somewhat ridiculous considering we had a number of mutual friends. But every time I tried to talk to her about it she would kind of... explode about other things. How damaged she was, or how fucked up life is, or etc etc etc. At the end of it, I usually felt terrible for even having problems. Like the fact she was a shit friend to me was somehow my fault.
My second mistake was not recognizing this for what it is. People who are familiar with abuse may recognize it as DARVO: "Deny, Attack, and Reverse Victim and Offender". It's a massive red flag that I genuinely didn't even see at the time.
Anyways. It turns out that behind what I considered unintentional or benign half-truths was an actual multi-month concerted campaign of actually lying about her life. I saw it, largely, but let her convince me that I was imagining things. You see, I want to believe people - I want to believe my friends. So when they do things that wind up hurting me, and I ask them about it, and they feign ignorance or tell me it's not what I believe, then I really, really want to believe it's a misunderstanding and that I'm somehow the one that's in the wrong.
That was my third mistake: for all intents and purposes I let her gaslight me, and didn't recognize it when it was happening.
This ended when she put me in a situation that had very strong emotional and even situational similarities with someone who was extremely abusive to me in the past. It all finally ended when she put me in a situation that ultimately caused me to suffer a fairly massive psychological break, and what I now recognize as fully dissociating for > 6 months. Even worse, at some point afterwards one of her close friends effectively tried to gaslight me - he actually successfully convinced me that what I saw in the situation wasn't true and that it was all in my head.
Except it wasn't.
I'm somewhat ashamed I didn't actually end the friendship there. I should have, but I didn't - mostly because I still wanted to believe that it wasn't intentional, or that she didn't actually mean to do it. So I gave her the chance to talk through things, or to somehow work to repair a friendship that I thought mattered to her. But that's the thing - everything that she did up until that point was a deliberate choice on her part, where she decided that whatever it was she was covering for was worth genuinely treating me like shit. And as much as it hurt at the time, she thankfully didn't care to try to mend things.
And that was my forth mistake: believing she was actually still somehow a good person, even after everything. I don't even know if what I saw in her - the good parts - were real, or just a byproduct of gas lighting. And I doubt I will ever actually know.
TL;DR: I learned that abuse isn't just a romantic relationship thing, and can happen in any relationship - including close friendships. Do yourself a favor and break up with abusive friends when you can - you're better off without them.