First of all, you need to like yourself (I know it's not easy, but you really need to at least work on this, no matter what), stop comparing yourself to others, absolutely you should not hate other people because in your opinion they look better than you (I know people like this, they will literally HATE you and blame you that you look "better" in their eyes, very annoying) and also you should stop looking for validation from others. Internal work is the best cure for low self-esteem.
I try to work on myself every single day. I am making mistakes, I am not perfect but at least I am trying to be better, overall as person, but not for others, only for myself.
And of course, it's good to go to the gym and keep diet, but do this more for yourself and your mental health, rather than for other people. And remember (I know, it's cliche) the most beautiful people are those you have good hearts and beautiful souls.