What should I do

[Marty Rogers]

A little off the topic...but I am wondering what state allows a 16 year old to ref JV basketball? In our state, one has to be 18 to take the NFHS (IAABO) exam before he is eligible to referee HS. Maybe someone younger could sit in on the clinic, but no certification until age 18.

This situation is akin to haveing very young people coach kids (especially who aren't that much younger than them).
They are great athletes, know the game, and have a great desire to coach. However, their age (teens, early 20's)
is against them in experience and maturity (I mean dealing with people/confrontations/officials/players/parents/etc).

Although the description of the trainer's behavior was unacceptable IMO (yelling SHUT UP), I agree with not sending the letter. Try to smooth things out (hard to do when you feel the guy is wrong), tell him you didn't meant to sound argumentative, and you'd appreciate his help perfecting your mechanics. Trust that (after 4 years of reffing without mechanics training) you have multiple bad habits that will be hard to break. Listen and learn.
Good luck, I hope you stick with it.