I’m at a wall here with my mare. I have had her for 9 years and we have had our ups and downs, I trust her and she has taught me so much. She is a machine, has never missed a lead change or looked at a jump. I have never had a problem as she has been my seeing eye dog for years now. For the past 5-6 months while at home, I cannot get her to hold her left lead canter on one of the long sides of our ring, we stay on the rail. She will hold her lead on both short sides and also on one of the long sides, but as soon as we turn the corner, around the middle of that side of the arena on the rail she always wants to swap. Does anyone have suggestions? We work with a trainer once a week and trailer in for lessons, I wish that I could afford more but this is all that I can do for now while the virus has hurt us financially. We’ve had her looked at by my vet and she is up to date with her yearly injections. She seems comfortable and has shown no signs of discomfort while being ridden. She will hold this lead on the lunge line fine with no swapping. I just don’t understand why it’s only one side of our ring going just one direction. Every other side, she is fine on the left lead, on the right lead she is fine on every side. It’s just that one side of the ring on the left lead that we can’t stay on the correct lead.
If you can’t do a full lameness work up I’d have the chiro out.
Edited:
If she went fine for 8.5 years this really sounds like a (possibly acute) lameness issue.
Does she do it loose in the paddock? Does she cross fire or totally switch both front and back leads?
I would personally investigate Hocks, Stifles and SI. But I’ve had some luck with a great chiro and chiro homework when I couldn’t afford digital diagnostics.
Is there a deep spot in the ring there? Something she’s looking at outside the ring that’s catching her eye funny? Is there something you’re looking at that is making you crooked?
Do you set her up to not do the change there? Maybe try a 10m circle to really drive that point home?
Clarification question - Is where you have your lessons a different barn than where you ride? That is the impression I get from your post.
Does this problem happen in your lessons too?
I have a champion swapper. 
Let’s say your horse swaps at B.
If you start at C and go 3/4 of the way around the ring, when you get to B, does your horse swap?
If you start at A and go just a little ways around the ring, when you get to B, does your horse swap?
In other words, is this about “my horse swaps her lead when she’s three quarters of the way around the ring,” or is this about “my horse swaps her lead at a certain point in the ring, even if we just picked it up 2 seconds ago?”
I make the distinction because if it’s only option 1, I’d be inclined to approach that as a soundness or fitness issue (she feels like she can’t hold it any longer and that’s where she gets tired- or YOU get tired, and you’re inadvertently cueing her to change her balance!) Whereas if it’s both or only option 2, I’d approach it as a behavioral or an environmental issue (it’s a habit/the footing changes there/she’s looking at the cows across the road.)
It might be interesting to take a few weeks and NEVER canter straight down that side on the rail. Canter down the centerline or the quarterline. Do big three-loop canter serpentines with simple changes in the middle of the ring. Do 10m canter circles down the length of the long side. Canter in shoulder-in down that side, or start at centerline/quarterline and ask her to leg yield to the rail - if you hit the rail early, do a circle rather than canter straight down the rail. If she’s bent around your inside left leg, it’s going to be harder for her to swap and you might be able to break the association with that long side by mixing it up.
I would also really think about what is in the ring, on that side, and in the corner you are cantering toward when she swaps. Is there a mirror, a viewing room, a pile of jumps? If her change is very auto and she doesn’t swap off at your trainer’s, something must be happening that she is taking as a cue to change - maybe you’re seeing yourself in the mirror and changing your balance, for example. Or if that side of the arena is open to the barn and you’re cantering toward the corner of the arena with the mounting block, maybe your attention is drifting right toward the barn plus you are closing your fingers a bit because you don’t want to gallop into the cluttered corner.
Can your trainer travel to you once or twice to help you get to the bottom of it?
I’ve definitely seen funny swapping being an early indicator of lameness but usually it’s not tied to one specific spot in the ring. I do agree that getting the chiro out could be worthwhile - there could be a combination of her being a bit off-balance plus something happening due to the ring/environment.
YES! Do what @173north said and report back, So hard to say if its physical on horse, is it footing, is it you, is it habit? Do everything BUT the thing that makes her swap for a few weeks and then go back to it and see what happens. You might learn a lot about your quality of canter by doing these things.
My A/O horse did that for awhile when he was younger. Same thing. Only swapped one direction, and only on one long side of the arena. It was me. Be very aware of your seat, legs, and hands. The horse is likely wanting to swap anyway (distracted, fresh, whatever) and you’re probably doing something to encourage it without even being aware of it. I like to sit crooked and have uneven hands. When my fresh horse would canter to the right (his weaker lead) and go down the long side by the barn, my weight would be a little too much in my right seat bone, he’d start to pop his shoulder ever so slightly and I’d of course try and fix it first with my hand (just the tiniest bit!) BAM! Lead change. Definitely happened more when he was fresh.
My recommendation would be to drop your stirrups and then do not go straight down that particular long side for a bit. Go halfway down, and circle. Circle and go straight down the last part of that long side. Etc. Always be thinking inside bend. And have eyes on the ground help you for a while until the issue isn’t happening anymore.
Does it also happen if you canter around with no cantact?
Was this horse taught counter canter before flying changes?
When no contact and no leg she’s splendid and does not swap. There are times when contact is ok.
If contact makes a difference, I would guess you may be uneven in the contact, especially if you are worried about it. Are you just turning the neck to keep the lead? Maybe think more about the straightness and ride the body of the horse, not the neck. I have one horse who will “tell on me” when I’m not riding him straight by swapping.
I agree with Greys. The horse is our mirror that shows every pimple and blackhead. The horse is telling on the rider. You need to thank her. She will make you a better rider.
I’m guessing it’s a rider thing also- I see you don’t have it in your budget for training rides but is there anyone you could ask to hop on your mare to see if the behavior persists? If it does, that’s more of an indication it’s a lameness or environmental issue than a rider issue- is she doesn’t then that tells you you’re cueing her to do the swap
