I've been working with a horse with a similar issue. She never has found her feet and was just a mess with cantering.
It's taken six lessons (1 hour each) to get her to relax into the canter and now she finally is learning to relax and use her four feet correctly.
I didn't use poles or anything other than a simple 22 ft line and a halter. We're in an open arena. I'd recommend an arena over a round pen. Simply because some horses feel too closed in in a round pen and can move more freely in an open space.
Anyhow,....so I just send her off and half circle stopped her by disengaging her hips and sent her out and change directions, repeat repeat repeat.
She would stay humped up and tense and would buck a lot and bounce around, rearing and bucking....because she just wasn't finding her feet.
But with so much go and stop (by disengaging the hips) and loads of hips over with her head flexed to the side (flex at a stand still then move her hips over then release and repeat a million times)......
Point is....the more HIP work that you do, you strengthen her hindquarters and you also help her find her balance.
Because when you disengage the hips correctly, the horse must cross the back feet and this takes balance.....because you are unbalancing the horse...he must relax his body to do this correctly and if he relaxes his body THEN he can find his feet to canter.
And I totally disagree with the age thing. Doesn't matter if the horse is 20. As long as there are no healthy issues, you can build this horse up and work just as hard as a young horse. Heck, there's horses that are competing at the NFR as barrel racers that are in their mid-20s.
Anyhow, the more HIP exercises you do (hip disengaging both sides, not just one)....the more you can help this horse find his feet.
Today I had an awesome session with the mare I'm speaking of. She's finally learned to find her feet and she canters on command....her owner says it's the first time she's seen her canter like that,....like a "normal horse"...and it's all because of the hip work.