How to deal with a negative self concept?
Learn to notice and identify your negative or self-criticizing thoughts. Become aware of how much you think negatively about yourself and diminish your importance especially to those who care about you or even love you.
Speak positively to yourself. Tell that internal critic to stop the insults. Instead, tell it you don't want to hear any more self-criticism. Practice positive and affirming self-talk.
Convert all "shoulds" into "coulds." If you feel that you should do something that is not particularly important to you, or is based on what other people think you should do...forget it. Challenge yourself to do only those activities that will reinforce your positive self-concept and strengthen your self-esteem.
Start exercising! Physical exercise is an excellent way to strengthen your self-concept. Physical exercise, in addition to improving one's body and health, nurtures positive thoughts and feelings because bio-chemicals released by the brain during exercise create a natural and healthy "high."
Learn to differentiate between the things you can control and the things you cannot. Remember, you have absolutely no power to change another person's chosen behavior. Recognize and use your personal power to change yourself and design and engineer your own life in the way you choose--leave changes in others' lives to the others!
Learn from your mistakes without self-punishment. "There are no mistakes, only lessons." Human beings learn through experience. The only time you encounter problems is when you fail to learn the lessons life constantly sends your way the first time they occur.
Learn from past errors and then let them go. Practice self-forgiveness and learn self-acceptance. If you engage in some or all of the above principles, you begin to uncover knowledge of who you really are and learn to like you.