My standard comment about depression is that depression is fear with nowhere to run and nothing to fight. But there is also clinical depression, which seems to be the brain’s neurochemistry stuck in this mode even if there is no obvious external cause.
Not being a doctor, I have no medical opinion about how much clinical depression can be affected by meditation and such practices, and I’d be loathe to under-play the importance of the biochemistry. I do know that when in depression, any suggestion that a person can affect their own internal states can sound like a condemnation. So…I don’t go there.
But for “normal” depression, it seems that our emotional states are controlled by three things: our mental focus, our internal dialogue, and the way we use our bodies.
I would suggest that you research people who mastered or healed their depression, and write stories incorporating whatever methods they used to do so. If you can make it believable, you might be able to convince yourself!