Are stress, frustration and existentialism considered cognitive distortions? I was given a list of cognitive distortions and ways to 'untwist' them. However I noticed the 3 big things that I encounter often were not listed. Are these considered cognitive distortions?
When you say cognitive distortions, I assume you are talking about cognitive biases: https://en.wikipedia.org/wiki/List_of_cognitive_biases

Stress and frustration could be thought of as experiences. For example, some forms of stress are associated with the experience of negative emotions, fatigue, and so on. They could also influence cognitive biases. For example, fatigue can lead to a narrowing of focus. Anyway, the point is they are not cognitive biases per se.

Existentialism is a philosophy. Perhaps you are talking about experiencing existential dread or something like that. But that also is not a cognitive bias, in that it does not inherently provide a biased cognition about something.