Mind believes, the way your body acts!! Believe it or not, doing 2 minutes of power pose can increase your confidence exponantially.
Make a list of things you have achieved in life. And in times of nervousness, read it out to gain confidence.
Everyone has their talents and strength, find out about yours and build on those things.
Set small goals such as walking 10k steps, not eating junk food for a whole day, etc. These small goals can make a huge impact on self-confidence.
Always remember, you are your biggest supporter. If you don't support yourself, then how to expect others to support you. Talk positive things with yourself.
Try something you are passionate about. If you work hard be toward your passion, it will make you happy and makes you feel confident.