Once upon a time in my past, I lived in a northern location where people jokingly spoke of "darkness poisoning" (or other words to that effect) during the winter months. The medical folks related to this as a real phenomenon, but I don't know what they called it. The season of long winter nights would always see a spike in cases of depression and worse.

Some people use "light boxes" or lamps that give off light in wavelengths mimicking natural sunlight to help manage their depression or circadian problems.

I'm not saying that's your problem, but I'm almost positive there are demonstrated neuropsychological connections between sunlight and brain chemistry, which you could research. So, when you ask, "Is there some actual, biological reason why I can be having a normal, even good day, and then, like clockwork, I feel miserable every evening and go to bed fighting off the urge to cry?", the answer is probably "Yes".