I own a 15 hand beautiful welsh mare, and within the last 6 months have been struggling with canter in the school. She works well in an outline and has a striking floaty trot, (very bouncy). I have no problem asking for canter as she is very responsive to the leg. She will generally canter 3 strides before falling out of canter into a massive, what I would call driving trot pulling her head down at the same time. I have tried everything... Squeezing her on once she's in canter turns into a gallop, especially down the long side of the school. Circles we can sometime make but I have to keep her very upright and bouncy and nine times out of ten I'll be back in trot in know time!
She is very long in the back, and am wondering if she is struggling and unbalanced.. Her saddle fits and have recently had the chiropractor out and there are no issues there, and she canters fine out on a hack.
Does anyone have any ideas that may help?
Pleeeeeeease

Also I forgot to mention two instructors have been on her and have the same problem.

Your mare sounds exactly like my daughter's. LOL It took years (and some frustrating failures) for my daughter to become strong enough in the leg, hand, and seat to keep her mare going, which is odd considering how very forward and free-moving the mare is. Onlookers oooh and aaah over the horse's beautiful movement and apparent willingness without realizing how much work it takes to keep her there. I have taken several dressage lessons on the horse, and despite my best efforts, I always have trouble with her breaking back to an extended trot. Sometimes she seems to feel claustrophobic indoors. That probably isn't your mare's problem, but it's something to think about. The other issue with my daughter's mare is that she has no self-confidence. She's an amazing and exciting event horse and 4 - 5-foot jumper, but she has to be "ridden" every step of the way. There's never a moment when she will just continue on a loose rein once the canter is established, and she wants direction up to and over the jump and on landing on the other side. Could your mare be of that same personality type?

Another thought... you say she drops into the extended trot with her head lowered. Is she also hollowing her back when she does this? That sounds like a great description of a horse who's moving "strung out" instead of on the bit. Could it be that you need to work more on up and down trot-canter-trot transitions and make sure she's rocked back onto her hindquarters to build her back muscles and show her that you want her reaching under and lifting from below? Does she lean on the bit or is she capable of collecting on her own without that support? I would never have believed that strong contact was necessary with any horse until I lessoned on my daughter's mare. With her (and she's been with us for 15 years and just turned 20) there's simply no other option.

I'm sure you'll work out the kinks over time. Keep the faith! LOL