Depression is a serious disease and very rarely can you "do better try harder" and actually conquer it. Treat your mental state like your injured back: you need time, rest, self care and possibly professional help.

Maybe this is "just stress" but "just stress" adds up over time to produce adrenal fatigue and other physical effects in your body. Maybe there's also some brain chemical stuff going on, in which case no amount of thinking or willpower will get you out. Needing medication, if indeed you do, is not a sign of your personal failure. Needing professional help of any kind for depression is not a sign of your personal weakness, it's a sign of your personal strength that you're willing to reach out when you need help.