Going back to the hairdresser is out of the question - my trust in all hairdressers is completely shot.
Relationships problems aside, I don't think this is a great take, OP. Any haircut could be fixed by the right hairdresser, and while finding one might not be an easy task, that's what reviews and previous appointments are for. You say it's been 3 months of you feeling miserable, that's not good and I'm really encouraging you to go fix it like yesterday. Hating yourself is never an option. Yes, you are more than your hair and you know that, but when you see something you dislike, these thoughts don't actually help. Fixing the problem does.
You can go completely different from your current style, maybe try wild colors (there are washable options and can be done at home), you can change your hair color in pictures first to see which options suit you best, then go for it (there is a free program to do so and many online apps).
For hair too short for your liking, you can get extentions, etc.
Whatever you do, do something! Hating on your current style and being miserable for months is NOT the answer!
Now onto your relationships.
for a while it was on/off so there's a lot of insecurity there already. Now he's posted something about being somewhere 'surrounded by beautiful women' and it's just made me feel even worse
Tbh that doesn't sound healthy at all and I really think you need to move on and never look back. The thing is, you can't be happy with someone when you're feeling low about yourself, and this man makes you feel bad. Doesn't sound like a foundation for happiness at all. Consider your break up with him a new beginning, it's okay to want someone back yet understand they aren't right for you.