Oh God endless stuff. Like you OP recently had a what I thought was a friendship end, and looking back on it with emotional distance was like having my eyes opened. Annoyingly, thought I was pretty good at spotting abusive behaviour, having had support after an abusive relationship for a few years.
With the friend, the whole dynamic was screwed in her favour. It still hurts a bit to see how much I was used; I wouldn't hear from her for ages, but then when she needed something, and had no other option, she'd call me. Or was it that she knew only I'd accept her crap? It felt at the time that it was because she trusted me, and that's what's so clever about these dynamics. They manage to make you feel privileged.
That's exactly what one of my abusive exes did as well. Made me feel special, like I was trusted above all others, that he did what he did to help me, that he'd forgive me for that mistake but I must never make it again, and could I modify that behaviour because it was becoming an issue... See how that spirals? My biggest error with him was believing his 'my exes were horrible people, the last one was crazy, she still needs me or she'll kill herself' stories. I had to interact with her a few times, and I was always polite but very cold. I feel bad about that now, as I'm pretty sure he was screwing with her the same way he was with me. I don't think a word he ever said was true.
That said, high insight is a wonderful thing. It's hard to see things clearly when you're emotionally invested, whether that's in a romantic situation or a friendship. Part of the way abuse works is by keeping you blind to it, and it's normally a small thing that is a wake up call and makes you think "What the hell am I doing?", and then the realization comes crashing down. Never be hard on yourself for not seeing it as abusive, because that is the essence of abuse.