Go hiking and try to accomplish a small task that is visible. What I mean by the second part is say cleaning a room. When the task is complete you have a very concrete and visible indicator that you have accomplished something. That's just one example if I'm super down it can be something as simple as brushing my teeth. Then you just try and keep that up. Conversely when I'm super down I just tend to go camping for a weekend or go hiking when I can. The peace of it allows me time to think about why I'm feeling down. Try to puzzle it out which will give you the tools to try and help yourself.