In my opinion, humanism is not really a theory. It is a necessary foundation to build rapport.

I use a form of existentialism most of the time. I prefer to work in the here and now, rather than to go back into past traumas and stories. I think going back into 'causes' can be a trap. Stories change every time they are told. Memories are always influenced by the present or seen through current lenses. It's just not possible to go back and view events neutrally. And a therapist can implant suggestions without knowing it.

So I think it is better to work in the present (existentialism) and build new stories of strength and purpose.

I'm actually more of a social constructionist, as I don't believe that our distress is internal (it is a social phenomenon) and I do believe strict existentialism is an ISE perspective. (inner subjective experience)

Of course there are different kinds of existentialism, I'm of the Frankl persuasion... within a constructionist frame!