If it's obesity that is causing your lack of self esteem then your only option is to get rid of your excess weight.
During this process, set yourself a lot of small goals. They don't all have to be weight goals, but it does help if they're related in some way, eg being able to walk around the block without stopping. Not eating chocolate for a straight seven days. That sort of thing.
Each time you succeed in reaching one of the goals congratulate yourself and record it in a diary so you can come back and read them in the future when you need to remind yourself that you can achieve positive results. It may seem silly, but it does work for some people.
These small successes can be the foundations for rebuilding self-esteem that was knocked down by failure.