This seems simplistic but it's a common model and based on years of self-analysis, journaling, reading and seeing therapists when affordable:

1. Old negative expectations (learned situational fears and the associated self-limiting beliefs and behaviors) lead to
2. A chronic attitude of learned helplessness / external locus of control / passive resignation, which leads to
3. A daily lifestyle and recurrent disappointing outcomes which "just seems to sorta keep happening to me", and would cause stress and low mood states in most people!

See how you can wind up with three problems to solve at the same time? That gets pretty overwhelming. It can form a self-sustaining cycle. One additional stressor on top of the subtle chronic issues and I take that nosedive down the rabbit hole known as "a major depressive episode".

I see it as more of a tricky mental game than anything else. I've tried meds, tried therapy, and I strongly believe that the right kind of talk therapy / life changes approach is best for me based on the evidence. "Treatment resistant" in my case just means that trying to find a really effective med has been disappointing, and that not all therapeutic models work well on me.

Insight-oriented psychodynamic approaches, mindfulness, and action-oriented cognitive behavioral strategies used together seem to work the best for me personally. Based on my theory of what my depression really is, that could be expected. Also, I can't stress enough the importance of the right kind of social support and encouragement (which is why I like PC so much).