I think, in my case at least, Misplaced Life Dissatisfaction is the main cause of existential angst. I've noticed a strong positive correlation between how happy I am at a given moment and how satisfied I am with my purpose in life (or lack thereof). There is a causal link, but it works the opposite way from what I first assumed: sadness makes me angsty, not the other way around. This despite the fact that my logical reasons for being angsty were just as valid no matter whether I was feeling good or not.

As with all biases, identifying the problem doesn't get rid of it, but it helps.