For me its the feeling of being trapped on a trajectory of inevitable decline. There are lots of permutations of this--aging is the most common one, where you know that no matter what you do, no matter how much effort you put in, you are going to get sicker, uglier, stupider, and eventually die.

A more pressing version of this feeling can be felt when you are diagnosed with an incurable illness, or, more generally, whenever you get placed on a set of failure rails which you cannot easily escape (e.g. social isolation becomes self-reinforcing and you feel trapped; unemployment becomes self-reinforcing as no one will hire you the longer you are unemployed and the longer you are unemployed the less likely you are to get hired).

It is a feeling most people probably have as they notice that they are aging, and one felt more acutely by those who get a permanent illness, age badly or fail to meet success milestones appropriate to their age (with regard to relationships, jobs, etc...)

So probably either that you getting your ball flicked