A lot of how you feel is how I felt at 23. The good news is that I'm now 38, firmly on the right side of confident when it comes to my brain, accomplishments, face and body. The bad news is there wasn't a quick fix and it's a long-term process - self-esteem isn't a button you press, it's something that comes ever so gradually and without you even realising. One day you wake up and think, fuck, I actually like myself, when did that happen? It's also an ongoing process - put me in certain situations and I can shrivel right back into my old overthinking, insecure self. I doubt anyone ever gets to Full Self-Esteem (and tbh the people who look like they have are also dickheads).
Scattered things I think have been useful.
Hold on to the thing you like best about yourself. The thing you know about. Your brain or your fitness or a skill you have. There IS something. If you can't think of anything, try believing your boyfriend when he tells you what it is! For me, I always knew I was book-smart. Fat lot of practical good it did me sometimes, but it validated me. Think of it as your trump card in life even when the rest of your hand doesn't seem like much.
Fake it til you make it. Cliched advice but it works, or at least it gets you through the week. Head up, look people in the eye, imagine what a confident person would do and then Do It. You can go quiver in the loos afterwards (I have done this).
Think about ego. There's a way in which getting tied up in your low self-esteem is actually being quite self-absorbed. You end up thinking about yourself, yourself, yourself all the time. Realise other people don't care. They're not judging you because they're thinking about themselves. Open your eyes and look at other people on the street. You'd never judge their faces, their skin or whatever as harshly as you judge yourself.
About looks. I also had insecurities about not looking like all the conventionally pretty girls when I was 20. I'm mixed-race and don't really look like a typical anything. Turns out the things that make you different are actually positives. Own what you've got and work with it.
About hobbies. I also get very frustrated when I don't pick things up easily, it's a poor character trait of mine. Try thinking of it as pursuing passions and interests, not being good at a hobby. The sport I play and the language I'm learning can both be very frustrating but ultimately I stick at them because I love them. Conversely if you don't love something enough to stick at it there's no reason that you should!