The Model and depression

Okay so I get, use, and LOVE the model, obviously, who wouldn’t?
BUT I’m not sure how it fits in with depression in certain situations.
I have depression, and it pops up from time to time, either for a few weeks or days. I find it’s difficult for me to track whether it’s my depression that has me down or if it’s my thoughts that trigger a depressive episode. For example, today I woke up with anxiety and oscillated back forth from that to depression. I did a thought download, and I for sure had mostly negative thoughts surfacing! I ran some models and was able to get to a neutral place with my intentional models, but still had lingering depression; feeling heavy, tired, and a bit hopeless.
I guess I’m confused about whether my thoughts trigger my depression and anxiety OR if my depression and anxiety (brain chemistry) was out of whack today and triggered my negative thoughts to surface?
Is there really any way to know?
Am I responsible for when my depression shows up?
Is it because I’ve been ignoring negative thoughts trying to surface for so long that they just come roaring to the top of my mind and knock me on my ass?
Are there preventative measures I can take as to not have depressive episodes, or are they just simply a part of my life?

LOTS of questions I know. Any insight would be great, I’ve been pondering this chicken/egg situation for a while now!