I agree with the be happy to be alone advice. That part is very important. However, a practical thing you can do, which you probably already know is hitting the gym.
It serves two purposes
It makes you slightly more attractive
It makes you aware that you are attractive
The second one is the more important one, but it can’t be attained without the first. You gain immense confidence by seeing yourself in a new light, as somebody that you yourself think is attractive. Going to the gym is also doable for most people, and while it is hard work it’s not difficult.
Ten years ago I started going to the gym. I gained a lot of confidence. I stopped going after a few years, but the fact that I know I can boost my attractiveness and confidence if I put in the work still makes me a happier, more confident person.