Some confidence comes from discovering what your body can physically accomplish.

There is a kind of confidence that arguments cannot produce. It comes from having done something physically hard and discovered that you could, and it settles into your sense of yourself in a way that positive thinking does not. The body provides evidence rather than encouragement, and evidence is harder to talk yourself out of.

Set a physical goal that is currently slightly beyond you and work toward it. A distance, a weight, a climb. What you gain from reaching it will show up in places that have nothing to do with fitness.