I wouldn't try and memorise anything before the course- but if you want to have a look at the sort of stuff it is, look at ICAO DOC 4444 especially chapter 5. It's publicly available. You will also realise that some/a lot won't make sense until on course.
The course/job is not about the theory- you need to have it, sure. However, it's the sim where you do the real learning and find out if you can do it.
As for the 'x' factor....
Maybe impossible to answer I understand, but would be interested in any thoughts.
My thoughts are; there are some talented individuals who are 'naturals' ( and by that I mean that they seem to have no trouble absorbing the information, are always ahead, and seem to always have the right solution first time), but the rest get thru by application, having the ability to accept criticism and adjust, self-confidence and the ability to perform under pressure, and sometimes just sheer doggedness. It goes a long way in this job.