I can't believe I'm admitting this after all my years of riding, but I'm still leaning out a bit at the canter on a 20m circle (me, not my horse) I rode in a synthetic saddle and it was off kilter after my canter ..... tell!!!!
Any suggestions would be appreciated by my horse and myself. Poor buddy.
When I found this happening a few years ago initially I thought it was me, but trainer had same issue. Horse had built up muscles on one side of it's withers and not the other - so mares muscling was "throwing me to one side and compressing the flocking on that side more than the other.
While working on exercises to build up the muscle on the smaller side I had saddle re-flocked to build up lower side (of course then had to have flocking removed once we'd fixed the issue). Saddler said it wasn't unusual and since he's also a trainer he suggested some exercises to build up lower side (sorry I don't remember them).
So first look at muscling of horse - is it even? If so you are the issue. If that's tha case are you perhaps "collapsing" a hip/leg and throwing your weight outside? Perhaps your inside hip is not forward enough (can't change that? then push outside hip back).
Or if you're brave enough (have a tough hide) you could post a video and see if someone on the BB can pin point the problem (or you might be able to see it yourself).
Actually, I think the "weight to the outside" problem is very, very common. People just don't know thy're doing it and most trainers either don't see it or address it. Watch the warmup at any show and you will see many who have their weight slightly outside at all paces and all lateral movements. Many of these folks are very good riders otherwise. The video that was up on this forum awhile ago contrasting Gal and Rath's riding of Totilas was a great example of this also. I had been riding with weight slightly to the outside in many movements for my entire riding career and have been to many trainers during that time. I had no idea and the problem was never pointed out to me until last fall when I started with a new trainer. I've been working on the problem all winter and am finally getting somewhere...but the road is difficult. Most everyone at this training barn has to receive reminders about weight shifts during their lessons......though I've never really seen it addressed to the same extent and with the same expectations by any other trainer. There's really no easy way to fix the "weight to the outside" other than trying, trying and trying until you can do it. More difficulty is thrown in when the horse is confused by your new attempt to ride correctly and turns, moves the shoulders in or otherwise responds in ways he/she shouldn't. I was dealing with this and resisting the idea of weight to the inside because it seemed to be screwing up my communication with my horse. I watched the Rath video previously mentioned and realized I had to dig in and get serious with re-training my body and my feel. Sigh. It's always something.