Depression seems to be caused by both external factors (such as crappy job) and internal thought processes (worrying too much, negative thoughts, etc) or a comb

Depression seems to be caused by both external factors (such as crappy job) and internal thought processes (worrying too much, negative thoughts, etc) or a combination. And some people are just more prone to depression than others. In your case it sounds like some kind of CBT, ACT or mindfulness therapy might be helpful. Hormones are well known to interact with the stress system and exacerbate depression. You make a good point about depression itself causing issues with (for example) your job, making it difficult to know what is the source of the problem. I think the best that people can do in this situation is to take a break from work, and think carefully about what they really want from life.