What mental exercises can I do to stop having such low self esteem?

I find talking to friends a good way to encourage me, if your friends don't encourage you then you can also try to make yourself believe what you want to be true (which is hard as hell believe me). You have to try to remember that 1) you are more attractive than you think you are, and 2) guys want a girl just as much as girls want a guy (I can confirm this because I am a guy and I feel the same way you do). try to convince yourself that those statements are true, because they are and they will make you feel better. It's not easy but once you do that you can start to feel better about yourself.