Stuff I avoid in people is firstly selfishness. There's a difference between putting yourself first and being selfish. It's important to understand it. Then you can spot selfishness and depending on how bad and if it's the first time or not you either stamp on it, draw your line in the sand that it's unacceptable or you walk away from the selfish person permanently.
Then being compatible, neither of you should be trying to change the other, or have a long list of things you'd like to change in them but you're putting up with. Someone whose company I genuinely enjoy. If I stop enjoying their company when they're no longer on their best behaviour then it's time to move on, because this is the real them and I don't like it.
Also watched for the signs of victim behaviour in myself. If I'm behaving like I'm being abused, I'm not doing that for no reason! So things like feeling I have to justify myself for wanting whatever perfectly reasonable thing it is that I want, is a red flag that something is off with the relationship.
Looking for the opposite, someone who doesn't put pressure on, who considers my wants and seeks out my opinions, who puts me first sometimes or when it's necessary instead of it always being about what they want. Someone who will make me a cuppa if I'm ill and check if I'm ok because they care. Not someone who'll say ok then see you when you're feeling better, and only check on me because they want to know if I'm up for going out yet. The ability to say No without any argument, bad feeling or justifying things. Saying No thanks its not convenient, shouldn't be hard work or cause me anxiety.
I decided to stay single and deliberately not be open to a relationship of any kind except friends for a while after leaving, because I knew I needed to recover and someone who wanted me when I was broken could either have issues themselves or be looking for an easy target.
I made an effort to curb people pleaser tendencies and live my life for myself, if others wanted to share it temporarily or potentially permanently that was fine but I didn't chase or wait and I took no shit. I wanted life to feel ordinary and trundling along fine not filled with drama, misunderstanding and argument. So if people bring me that I disengage.
I wanted to be myself, compromising on who you are causes stress, so now I'm take it or leave it. We're all different, we're not all going to like each other and if someone thinks I'm not for them, that's fine. If I'm sad about that, it won't last forever and it frees up space to find someone truly compatible. I believe compromise should only ever be for minor things like which pub to go to or what colour carpet to have in your shared home. The major stuff either works or it doesn't and trying to compromise causes problems and resentment.