Concentrate on other people.
Get out and volunteer to help people - digging the garden for an elderly neighbour, helping in a charity shop, - there are loads of opportunities to stop thinking about yourself, and you will very quickly get a nice inner 'glow' that comes with doing a good deed, and it will boost your self-esteem. We are all guilty of of being self-obsessed. Get a grip on it and you will feel so much better.
Concentrate on what you can do, your skills and aptitudes. Try to focus on those things and not the negative aspects of your life. It won't happen overnight but eventually you will learn self worth.