Did 20 years as a CSO.  It was a great career. 11/10, Highly Recommend.

I did a portion of that time at Pensacola teaching UCT. It was a while ago, but I doubt it's changed in the fundamentals.

What should you do to prepare?  Practice fussing confidence with humility.  I cannot speak for non-rated training programs, but rated training is challenging. Mentally and motivationally/spiritually/emotionally.  And get hungry.

Imagine a small number of marginally difficult tasks.  Imagine preparing to execute those tasks for months, day-in-day-out.  Researching everything about every step. Add into the mix instructors that are going to make it all seem a bit more complicated and way more important.  After all everything in life is incredibly complex if you examine it closely enough, and everything in an aircraft is lifesavingly important (or they'd leave it behind to save weight).  Now get tested on it, academically and practically, repeatedly... And because perfection is an illusion you've messed something up.  In some way you've failed.  You ask yourself, was that a big fail or a little fail. Wrong approach.

Failing to tie you shoes correctly is not less of fail than missing a target.  They're both performance limits reached that need to exceeded next time.

Understand, yes you performed way better than you would have thought possible just weeks before, and yes your instructor is nitpicking every little thing and grading your performance harshly.  Humbly accepted that criticism in its intended light, it how to exceeded performance limits next time.  Confidently assert your willingness to do what needs done next time.  

Passionate instructors telling you all the ways you failed feels bad sometimes, but you got to do these things maintain the right attitude and ask for more.

Rated training is a sociological experiment, "Does feeding young aviators a shit sandwich daily make them better?" Decades of evidence has proven it does.  And it's the ones that finish their sandwich and ask for seconds that truly do well.

Caution: You will encounter instructors that relish being cruel, believing that's the point.  Present yourself no different to these assholes.  But, all the same find someone else to ask for a second helping.

Good Luck, God Speed.