The best source of confidence is internal. looking inside yourself. too many people llook to outside influences and these can be tricky at best.
The best way to build confidence this way is to learn to look at your victories in life and quit focusing on failures. We all make mistakes, but if you dwell on every negative thing that happens, you will always be finding more things to dwell on.
All great things come from within first and then are externally experienced. So, the best sources to do this is through keeping a victory journal, where no success is to small to keep. If you are trying to lose weight and you lose 5 pounds, celebrate it in your journal.
The other source is meditation. spend 15-30 minutes a day in relaxing meditation, letting go of all the negative thoughts and focusing on just being in the moment.