It's a mindset.
I used to be depressed; nobody knew and nobody asked. I didn't tell anyone, I never got diagnosed. In fact this happened to me multiple times in life.
I used to feel low, so low, that I was in situations where I didn't feel like I belonged there. For some reason I kept moving myself in these situations to be there, do things such as meeting friends and exercising! I kept moving myself into these positive situations. Having moved myself more, I eventually got out of the vicious cycle of depression. The alternative would have been to stay in bed, as I felt like doing that. I never felt like exercising or meeting friends, but I just did it regardless.
If you feel tired or depressed, the worst thing you can do is stop moving. Do some exercise, go for a run, walk or call a trustworthy friend to hear you out. By doing this, you will always feel better after it.
Now I will never feel low, because I am aware of how I feel and I can generate my own energy to lift myself up by always moving myself even when I don't feel like it.