I know this always sounds like some sort of "goes without saying" non-advice, but in my experience you should never underestimate the impact simple things like dressing well and generally looking out for yourself (even on a merely superficial appearance level) have on your self-esteem and how others perceive you.
On a less superficial level, you need to feel good about what you're doing, i.e. your job/your academic studies/whatever, your hobbies, your social life.
Take your time to see whether or not you're content with where you are, identify areas where you would like to change things and how. I realize this is easier said then done. Sometimes it will require outside help, e.g. coaching, but sometimes even a word of advice from someone you trust might be enough to inspire you.
On a less superficial level, you need to feel good about what you're doing, i.e. your job/your academic studies/whatever, your hobbies, your social life.
Take your time to see whether or not you're content with where you are, identify areas where you would like to change things and how. I realize this is easier said then done. Sometimes it will require outside help, e.g. coaching, but sometimes even a word of advice from someone you trust might be enough to inspire you.