She hasn't been ridden in 3 years so she has no muscle tone. Just like a baby she's lost the nack of balancing with a rider. Stop cantering for the moment and work on learning to rebalance her through halt halts (HH).
A HH can be used to slow or just rebalance the horse. (Horses get faster in the gait when they fall onto their forehands - nose 0 thus when you rebalance them you're shifting the weight back onto their hind legs.
When the horse is moving along (say at a walk) then starts walking faster (returning to the barn/sees another horse/etc) ask for a halt. You do by keeping BOTH legs on (slight squeeze), then as horse goes forward push BOTH stirrups straight down (rider needs to be straight up and down NOT leaning forward or backward) while at the same time squeezing the fingers on both hands to indicate a WHOA. (Using the voice to teach a stop can help).
Once you get a halt where BOTH hind legs are straight down (not out behind the tail) then you use that same technique to teach the horse to HH. To do the HH ask for a halt, then when hors starts to stop at last minute ease reins forward about 1/2 inch and allow (do NOT push) horse to continue forward. Repeat often.
Once you can do that then everytime you ask for a transition (up and down) do a HH then ask for the transition - walk into trot, trot into walk, trot to halt.
After 6 months of doing that (maybe less if you and the horse learn quickly) the horse should have the muscles where at the trot you do a HH, ask for the canter. IF you did the HH correctly the horse stepped underneath itself with the hinds legs and the canter will start out slower (than ity does now) since the horse will be balanced. After a few strides horse will loose balance since it still needs muscle, so HH then transition BACK to the trot for another 6 strides, then HH and transition back to canter.
Repeat with lots of walk breaks where horse can stretch nose to ground on long rein to relax and stretch out the muscle it's building. Soon (if you're consistent) horse will be able to canter balanced as slow or as large as you want.
Dressage is for Trainers!