How severe can seasonal affective disorder possibly be? can it mimic moderate-severe depr., anx., and depersonalization?

Yes: Seasonal affective disorder creates anxiety and depression due to the lack of sunlight in the winter. Your doctor can refer you to a psychiatrist for medication that can help.

Depersonalization?: What used to be called "seasonal affective disorder" is now diagnosed as depression "with seasonal pattern," typically in Fall/Winter when sunlight decreases. So yes, it can be full-blown (severe) depression. But depersonalization is not a depression symptom. If you are experiencing true, depesonalization, something more/else is going on. See a professional for a thorough evaluation.