My depression used to last for months and it was moderate -- that is, until I began to cycle very rapidly, which resulted in my depression lasting 2-3 weeks and being very severe.

I know what you're going through, though. It sucks. I agree with gina_re that you should talk to your pdoc and your therapist. There are some fast-acting antipsychotics out there that might be able to pull you out of your depression. (For example, Latuda tends to work within 1-2 weeks.)