Hmm... the best idea would be to just ask your instructor exactly how to get him to move. What she did exactly when she got on his back. It's not your fault at all. Your doing everything you know how to do. We all have those horses that just give us a hard time personally. Maybe not for everyone else though it seems. Have you tried a crop on him? If you don't ride him with one, take the end of the reins (the bight) and wack him with it like you would a crop. Basically the same but without carrying the stick.
If she said your stirrups are okay, tell her it would just make you feel more comfortable if you had them the way you want. Your the one riding the horse, you should feel comfortable. It can sure be annoying riding with stirrups that are too long for you. Even if they don't look like it on the ground.
With your legs, try using your ankle bone to give him a quick kick. Give him rein too.
Also, the horse probably knows your instructor well and knows that as soon as she gets on him she means business. You know what I mean? He's taking control of you and you have to show him your boss.
Overall though I think a crop would be best for you right now. :) Good luck! I know those problem horses can stress you out a bit! They help you learn a tonn though! Never regret the problem ones:)