Any question answered...

I'm writing an application essay for a master's programme, and I want to convey to the reader that the particular experience I am detailing was profoundly life altering. "Transformative" doesn't sound quite right to me - maybe it's a North American vs. European thing.

That's certainly possible.

Transformative, in my non-dictionary-like mind, and philosophic-centric perspective, is a process which can be discussed and understood (albeit abstractly), rather than a one-off event in one's own life (which would be, instead, a life-altering or life-changing event). That is, home-schooling children causes a transformative change in the student that public-education doesn't. However, on my trip to Spain, I had a life-changing experience on the beach when I ate way too many mushrooms.

But I can understand you wanting to avoid those words (life-changing/altering) as they're a bit of a cliche. Just don't use the wrong word! (which, obviously, you're making the effort not to).

^ this is all BS, but it's an honest response. Hopefully helpful?