When you are pursuing a dream, or a challenge. When you are seeking, trying, and working hard, there are moments of depression. For me, this passes quickly, because I have a sense of self and I get over my bad mood by knowing that if the problem won't kill me or ruin my dream and plans, ( maybe something will delay it), but I try to focus on the steps I have to take to have the things I want. You have to choose to be happy, to be positive, and push yourself to claim your goals and pursuits.
If you do something nice for someone else, it helps too. Sharing your experiences and listening to others stories and experiences can also be inspiring and get you out of your funk. I usually call my friends who are chasing their dreams too and we talk about it. The Good, the bad and the ugly...but also the success we sometimes don't stop to notice, always rushing for the next thing- but when you take a moment to notice your acomplishments and what you are...right now...you can also find happiness in the NOW.