As corny as it sounds, just have some belief in yourself and your abilties and take pride in your achievements. Don't compare yourself unfavourably to other people unless it's in a constructive way, e.g. if someone's got a promotion because they are better qualified, rather than be miserable because you're telling yourself that person is 'better than you', go and get that qualification yourself. Confident people generally instil confidence in others and (unless they are over-confident, in which case they just pee you off) are nicer people to be around. This means you get positive responses from those around you which in turn boosts your self confidence further. Have a look at the friends/colleagues/acquaintances around you who you consider to be self confident and I bet you they're not necessarily the most successful/attractive/wealthy people you know? They are just people who believe in themselves and have a healthy perspective on life.