If this is typical practice, then without a doubt your flight
instructor is unprofessional and violating the FAA's guidelines on the
fundamentals of instruction (FOI) Never, ever, ever pay $50/hour for
someone to treat you like crap.

Find a new instructor, preferably one who enjoys instructing. You are
a client, he is providing a service and under no circumstances should
you be reduced to anything less.

Incidentally, "slamming" the throttle, or any other aircraft control is
a bad idea, and on more than one occasion results in broken cables,
linkages, and can even be the straw that breaks an engine, not
something you want to have happen with 20 hours in your book.

Check our website, there is plenty of information on CFI's, jobs, and
the basics of VFR instruction.

www.pilotweb.org