How should I prepare mentally for the USAF PJ's?

In 7 years, 3 years of high school and 4 of collage, I am going to start the Pipeline for becoming a Pararescueman. There is plenty of advice on how to train physically, but I have been told that it is 90% mental. How can I prepare for the mental aspect of it? I was thinking of going through Marine Boot Camp just for fun, then dropping out (just to get used to the stress and the fast pace of military selection and training), but is that possible, and would it be helpful?