Uncertainty

Because I live with depression, I never know what's coming next. That's true for everybody, but there's a wider range of likely scenarios for me than someone who doesn't have a mood disorder.
Someone who is neurotypical and feels irrationally angry at the world for a few days and notices a degree of lethargy which is preventing them from completing their planned activities might wonder if they're getting a cold or the flu. They might not even notice the impact it's having on their life, or they might attribute it to the extra work of keeping warm in the extreme cold of this winter. A week later, it will be forgotten.
When I feel myself filled with rage for no particular reason, and I end up in bed before 7pm two nights in a row, I get scared. Maybe it's just one of the reasons that would come to mind for anybody; it could be a transient illness, the weather, or the reaction to an unusual level of stress or extra work.
But maybe it's more. Maybe it's the start of a depressive episode and my life is about to fall apart, or - in the best case - get a lot harder.
Or maybe it is one of the "normal" reasons for feeling this way, but it's going to trigger an episode.
I have resources. I'm not alone. I've spent years and thousands of dollars working on understanding this problem, identifying its patterns and pitfalls, and learning what I can do to minimize damage. Maybe my actions will even prevent a full episode from developing.
Or maybe I'm just having a bad couple of days and all will be back to normal next week.
I never know. And that's the scary part.