First of all, become aware of the negative things you are telling yourself and try to see if they are backed by facts. Try to be objective. When the same thought is thought over and over in the brain it strengthens the neural circuits related to the thought. Try to change the pattern.

Can you try to find a local self-esteem class? It might be good to be around people with similar problems.

Buy a book and go through it and do some of the suggested exercises.

Hope this helps.