> The other day I was crushingly depressed and had been for a week
In my regrettably extensive experience, if you feel bad for a week then you can count yourself very, very lucky. I can't see what you describe as being depression, although of course people are very different (had GFs who had it bad for a week or so at a time every month).
> I [...] hit my heavy bag for twenty minutes and came out feeling fine
Depression made me exercise compulsively, to the point of damaging myself. It never made me feel better so I don't know why I did it, but I had to, and I understand this is common. Twenty minutes of exercise and you felt better? It's impossible to equate that in any sense with what I had.
There's a big difference between feeling really crappy and knowing you'd literally be better of dead, and being in that state for years continuously. That kind of things hurts inside, worse than physical pain (I know both).
> my lived experience is a superior epistemic method here
Hmm. Good for you.