Well, I rode a different horse in a different dressage saddle a few months ago (FEI dressage horse so he had a BIG trot!) and found that I didn't struggle at all to keep my position or stay with him. My legs and abs died because I'm not used to doing much sitting trot at all, but the coach I was riding him under didn't have any complaints about my position or lower leg at all beyond a couple of reminders to keep my heels down, and to stop leaning forward when I ask for the canter (bad habit of mine!!). She kept yelling at me about my dreadful circles, but by the second lesson she left my position alone entirely... except for the canter thing which I honestly cannot break!
So, I thought it was my horse, because his movement is different to a big floaty warmblood (rather flat and he can be very bouncy)... still worked on myself a lot, but had issues in BOTH my saddles.
Well, the other day, I was riding bareback, and suddenly I realised that I was sitting the trot, my lower leg was staying still, and I was having no trouble staying with my horse. Saddle back on, no stirrups... BAM can't sit trot. Saddle off, can sit trot. Switched saddle to my jumping saddle, no stirrups, can't sit trot. Saddle off, CAN sit trot.
Starting to wonder if my saddles are the issue here? The dressage sits really nicely on him and doesn't move at all, but it's a synthetic suede bucket seat type saddle and I have friends of similar body types who cannot sit the trot in that particular model of saddle. The jumping saddle is CAIR, and bounces, and nobody else who's ridden in it can sit the trot in it.
Borrowed a friend's saddle, also a synthetic suede deal but with a flatter seat. Could sit trot easily!
...I think I need a new dressage saddle, but meanwhile I'm celebrating the fact that actually, I can sit the trot, it's NOT me, and it's NOT my horse.