Is it allowed according to the halocha to do a hypnotherapy session where past life regression is done?
I'm going to answer in a round-about manner.
I asked 2 of my Rabbeiyim about the possibility of past-life-regression and one said that he heard from a Gadol that it's not possible to access anything about the soul from before birth (I understood he meant by non-mystical methods, since we know from stories of mekubalim that were able to access such things).
I had trouble with this approach since some of the documented instances of past-life regression contained way too much corraborated information to be imagination alone.
I spoke with Rav Yitzchok Breitowitz about this and he too heard from an "Adom Gadol" that's it's not possible. I asked him about the documented cases and he wasn't sure.
I asked him if we could reconcile to 2 things by suggesting that a person who does a past-life-regression isn't seeing their own past-life necessarily (which would require knowing and accessing their own soul's history) but they are accessing someone else's real life memories (which does not require tapping into the "correct" soul). This then would be an instance of telepathy, which Rav Dessler says is definitely possible.
Rav Breitowitz said that that is a possibility, and he added that it would be a type of "chibbur nishamos". So that the one doing the past-life-regression, may be connecting with a real character in history, and yet the person's own individual soul history would remain unknown.
From these 2 rabbonim, I took out that it is permitted (no less than an imaginary story for the sake of therapy) but that it's limits should be recognized.
The way I understood it is that even if you are connecting to a real past life, it would based on your current life (you'd be connecting to someone with a similarity to your current life's challenges and situations etc) and so your past life would remain a mystery.
If I may add one more point that I took out of this: what happened to you in a past life is irrelevant to your current life. Meaning, even if some things from your past life brought you to your current situation so that you can work on whatever you need to work on, that mission or whatever you want to call it is here now, and you have all the information and tools to deal with it, because Hashem gives everyone everything they need to fulfill their task in life. (And I know that past-life-regressors may disagree with me because they'll find current life problems rooted in a past life, but I'll disagree with their conclusion, and I explained above).
So, in a nutshell: Permitted but do it with a grain of salt.