Are you as confident as you would like to be? Not many people would answer "yes". Confidence can be relative; you can be very confident in some areas and totally insecure in another.
When you are experiencing a lack of self-confidence, take a moment and consider what you are comparing yourself too.
If you are good at something, say running, you are feeling good about yourself and your confidence is high as you are leaving other runners behind. Now put an expert next to you, someone who runs everyday and in competition, then your confidence will dwindle a little.
When feeling insecure, take a break and ask yourself 'Who am I comparing myself too?" Accept what you consider short comings, work to improve them, and that will boast your confidence.
Have a confident day.