Sorry I didn’t answer your question about course length. Yes, PgDip will be shorter, generally though (as I understand it, I might be wrong) PG dip is the same as the masters but does not include the dissertation.
At my uni, the Masters I did was 12 months Sept - Aug, the PGdip was run simultaneously Sept-June, and was the same except no dissertation.
Some people started out intending to do the PGDip, and others started out doing the Masters but failed a module, so got a PGDip instead.
If you would like to give yourself the best chance of getting onto a psychology doctorate, I’d definitely go for the Masters, as the in depth research training is really important - especially for people like us with Arts degrees who have not done scientific research before. You are not likely to manage a doctoral research thesis when you have never done any research in the discipline before.
As I said before, my suggestion would be to do the Masters part time over 2 years if you are worried (or, start full time, and drip down to part time if you’re finding it too much).
If you want to do some studying before the masters, maybe read up on imposter syndrome, perfectionism, growth mindset, self esteem, self compassion etc.