Glad to see this thread as I have the opposite problem (lose my stirrups in a hunt saddle, feel great with a long leg ie dressage or western). I LOVE the suggestions for loosening the ankles and the visual to get the big toe to touch the horse. I am self-taught, and found that though I rode Western for 99% of my riding career so far, I tended to have a long stirrup but with a sort of hunt leg, if that makes sense. Heels far down, toes a bit pointed out, etc. It's a lot of work to get loose ankles and a "drapey" leg, but I find it a lot more comfortable, especially since I have bad knees.
I will also attest to the lack of core strength being a contributing factor. Being 7mos pregnant right now, sometimes I feel like a big wet noodle trying to sit the trot/canter of our long-backed, steep-shouldered mare.
All in due time. :)
I have said what I have said. I have not said what I have not said.