It sounds like she is scared and just can’t calm down. (I could be wrong, but that’s what I have gathered from reading your problem) So here’s my 2 cents worth…
Ride her at a walk just around the area of the arena, and reach over, pat her neck and talk to her, pat her rump, do this until she drops her head and is just like “ok I’m comfortable with you up there”
Then like the next day (I’d say do it that day but it might be too soon) saddle her up again, and review the walking lesson, go around the arena like 2 times, then ask for a trot and do what you did at the walk with the trot. So basically what you’ll be doing is riding around the arena and patting both sides of her neck, and patting her rump. So then do that at a trot until she’s ok with you moving around and being up there.
Then the next day, you’ll review the walk (2 times around 3 if she’s forgotten a bit and doesn’t seem calm like) and then the trot (2 or 3) and then ask for the canter. She might freak out again, but when she does just go back to the trot and work on that for a while, if you have to, go back to walk and then move up if she’s ready. Just keep talking to her and telling her it’s ok, and pat her and what you did during the walk and trot. She might break the canter, but just ask for it again. If she’s freaked out and you’ve gone back to the walk/trot, then once she’s cooled down, (if your walking ask to trot and move to the canter slowly) you can ask to canter again. Chances are she’ll freak, but then you just repeat what you did before.
Try to end your lesson on a good note and if you have to end it walking/trotting, heck that’s fine. What your trying to do is gain her confidence to be ridden at the canter, and to get there you gotta build her confidence. So just try that, and it might take more than 3 days, just give her time. And I wouldn’t throw any poles in when your doing this work, so that way she can focus on being comfortable with you. Good luck and keep me updated. if you have any more questions feel free to ask! 🙂 🙂 🙂
~If you come at it having only 15 minuets it will take all day... If you come at it having all day it will take you 15 minuets~