A first step is to try to understand where your problem comes from and all the ways it manifests itself. The reason that this is important is that your approach to addressing the problem varies depending on the more specific nature of it. I'll try to give a broad overview here:
I. Early Experiences: These range extremely broadly from the one end of lack of affirmation/affection to another extreme end of trauma and abuse. Messages that we receive early on affect the development of our core beliefs about ourselves
II. Development of Patterns of thought: which range from the very deep core beliefs (I'm no good), to the conditional thoughts or "rules" we live by, "If I work hard, then I'll be happy," to fleeting automatic thoughts "That person didn't say hi, they don't like me." How we feel about ourselves is heavily influenced by our beliefs. These are not changed easily by just thinking "positively."
III. How we function as a result of our beliefs --again a wide range here from just not liking one's self much to significant clinical depression and every problem in between including addictions, eating disorders, anxiety, etc.
What to do? Therapy would be your best option --where you can spend time identifying your significant life experiences and look at what your beliefs are at all three levels. You may need to process/resolve some early hurtful life experiences.
Cognitive Behavioral Therapy (CBT) is very useful for this sort of thing. CBT examines the interplay between beliefs, emotions, behavior, and feelings in one's body when in a particular situation. Intervention in one area will affect the entire system. With CBT one does a lot of work with identifying beliefs and making counter or alternative beliefs to the irrational ones. These are then tested out in real lifesituations in the form of "experiments." It is a highly effectiveapproach for working on changing how one views one's self. You can read more about it here: www.mindovermood.com andhere www.nacbt.org/whatiscbt.htm
If therapy is not an immediate option, then try buying the book at mindovermood.com --it's a work book that you can try on your own.
Please feel free to follow up with me.