lack of insight bipolar

Hypomania, bipolar II depression for 17 years. Is depression caused chemical imbalance or external event.
Can a tramatic event, although it seems manageable in the time cause depression. Or is a chemical imbalance (low sertonin) finally lead to depression, with or without external causes. Is bipolar II hypomania bipolar mixed state difficult to diagnose? Can a person with the above suffer from "lack of vision" much more common with Bipolar II. I doubt an event 17 years ago can cause a slow but steady slide in quality of life issue. I'm in therapy. With talks revealing with old friends has changed my behavior (slow), but still nearly two decades! Is this possible?
I made a lot of research on bipolar and really have not found a constant cause for depression to mania. But anit Antidepressants do not work in depression, because in a study that found a placebo was more effective than the antidepressant. So it's not especially chemicals. So it is very possible that an event 17 years ago could have caused it. It is called diathesis stress model. You may have predisposed to bipolar disorder, but had a supportive environment and nothing to provoke it, but this has upset everything.