Existential psychotherapy has its roots in Philosophy and the many great philosophers who have lived and continue to. Epictetus for example talks about existential experiences in an accessible way: we can only control two things -- our thoughts and our actions. Nothing and no one else.
"There is only one way to happiness and that is to cease worrying about things which are beyond the power or our will."
There is much that has been written about the existential approach and if you wanted to find out more I'd suggest researching goodtherapy.org: www.goodtherapy.org/learn-about-therapy/types/existential-psychotherapy or books written by Yalom (www.yalom.com -- start with Love's Executioner) along with van Deurzen or Spinelli.
"A curious thought experiment. . . Nietzsche's message to us was to live life in such a way that we would be willing to repeat the same life eternally
Irvin D. Yalom