Albert Bandura, on your reading about, for this week, has done a lot of work on the concept of self efficacy which is essentially the academic term for believing in oneself.

True confidence comes from the gradual accumulation of self-efficacy over a long period of successes.

Each one will lift your mood, as will the mere fact that you've tackled and achieved some concrete goals; by doing so, you boost your feelings of self-efficacy, which can boost happiness.