I seem to follow a similar pattern too, so I'm reassured that it isn't just me.

I find that the deep, dark days are linked more to anxiety. So the racing thoughts, the agitation, irritability, fear and hopelessness link to high anxiety that seems to escalate without an obvious trigger. I have just got a prescription for diazepam, specifically for these extra dark days and it seems to have helped, one dose has given me a week of relative tranquility, my mood is still low but I can function instead of crawling under the covers and hiding.