You've got a sore backed horse. Pain is causing the "attitude."
First probable cause: rider position. You've got you butt back on the cantle and your feet on the dashboard. This puts 70-80% of your body weight on the back two points of the saddle, vice distributed over the bars.
Second probable cause is saddle fit. I say this because ill fitting saddles are found on 80% +++ of horses regardless of breed or type. If the saddle fits poorly it grossly increases the pain the horse is feeling.
Get a riding coach first and correct your seat. Fix the saddle fit as required. These should put you on the road to success. If the vet did not find anything (and they are a competent vet) then calling a chiro is wasting money because there is nothing to fix.
Good luck in your program.