Low self esteem so I can’t have a bf (help)

I’m 18 and never had a bf. Guys have tried asking me out, but I turn them down because, even if I like them back, I feel like they can do better than me. (EDIT: let me clarify I’ve only officially been asked out once and maybe 2 guys who have tried flirting over snap. So not much happens) I can’t stop thinking about everything that’s wrong with me, how they’d be better off with someone else than being stuck with me, etc.

I mean clearly I just have low self esteem but Idk how to stop this type of thinking. I really want a bf and I do regret turning away 2 of those guys, but again, they’re r way prettier, funnier, overall nicer girls out there so why would they want my ugly ass, ya know? I just wish I was different and I’m getting really stressed and depressed about it. I’m legit gonna be that old cat lady who dies alone 😫 can anyone give advice on how I can get my self esteem up and just overall tips for getting a bf, etc??