Longer stirrups versus shorter stirrups?

My instructor insists on having my stirrups long. I thought they were long before and often struggled to not lose them. As soon as I got to the point of having a full lesson without losing them, she made them longer once again! One more hole longer and I mine as well not even have them! I ride English/Dressage... show more My instructor insists on having my stirrups long. I thought they were long before and often struggled to not lose them. As soon as I got to the point of having a full lesson without losing them, she made them longer once again! One more hole longer and I mine as well not even have them!

I ride English/Dressage and have a decent seat...I can stay on long enough to stop the horse if I loose my stirrups, even at a canter. However the longer stirrups make me feel... insecure. I am riding a light-draft horse with very powerful gaits. Her gaits are gorgeous but her trot, even when posting, is strong and her canter almost feels like a charge! Sometimes those shorter stirrups make me feel more secure.

Some people have recommended I shorten my stirrups until my seat improves. I can stay on pretty well, but sometimes I pinch my knee, my leg comes forward too much, and I get a chair seat. My trainer does the opposite of what others have recommended- she lengthens them. My hip flexors were very sore because I felt like I had to constantly reach for them while posting AND keeping my heel down AND my leg back.

My instructor is successful and I do trust her very much. But sometimes her methods seem opposite of what others would do!

What do you think about this? What are the pros and cons of longer and shorter stirrups?
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