I mean I was just an absolute mess of a person in my 20s, a bitter 4channer who struggled to attain enough self-awareness to identify my problems, so I guess
Basically everything. I got serious about therapy and started seeing an ADHD specialist, and got back into physical therapy for my chronic pain. I took an online social skills group coaching thing to help me find my rough spots in my personality and my conduct during conversations, and build up my confidence. I started going out to karaoke with my coworkers and was surprised to find they were impressed with my singing. I got back into school to challenge myself to do things that were still frightening to me, like seriously pursuing my dream of being an author and exploring the possibilities of voiceover work. I got onto a plan to eliminate my medical/credit card debt. Started taking better care of myself, got a basic skin care routine. I grew out my beard and started taking good care of it, brushing it, oiling it, etc.
Since then I've practiced the basic advice they tell you: be polite, be appropriate, be interesting, be funny, present yourself well, and be honest. I've caught the interest of a few women and gotten started on figuring out what the hell I'm doing wrong, and lately really figured out how to learn from my failures and take the positives in stride. The last girl I went out with treated me poorly: stood me up on our first date, didn't text me for a week, then had me drive out for an hour to meet for drinks only to ditch me after an hour with some excuse, and ghosting me. Quote from her: "I don't ghost people." But before that she had made it very clear to me that she liked me for my body as well as my personality (I guess just not my finances) and that helped me finally dispel some serious body image issues I've had. The other day another girl I thought I'd screwed up with reached out apologizing for not getting back to me sooner and refused to let me downplay what she'd done. She's still bad at texting me back but I figure maybe this weekend she might have a chance to get back to me, but even if I never hear back from her again, I know I can probably find someone.
I had to navigate my way out of a lot of toxic core beliefs and keep myself alive against the urge to annihilate myself, but ultimately I had to learn to believe it's possible to improve and grow and find the right person in time, and accept that pain, lots of pain will be inevitable, but you have to choose to give it meaning and keep going. And most of all, have patience, and protect your own heart. Go into things without expectations. Accept that good things might happen or they might not, but don't pin your hopes on a certain outcome.
TLDR: Keep growing, keep learning. Self-improvement and confidence are in and of themselves attractive, and as you grow you will become a more attractive person overall to boot. And don't allow the loser mentality to creep in. Know when to take a break, but don't give up and don't assume you'll be alone forever. Dating sucks ass, even for hot people.