My pony was just like this and is still some days and is just starting to losen up and accept my leg. This is what worked for me and her; I really grew my bond with her and did a lot of bond exercises to get her to really trust me and know that I would never ever hurt her. Then slowly through circles I would apply my leg as a leg yield to support her through the turn. I just started slowly building up the amount of leg I used through the turns. This should also eventually help with getting her to lengthen her stride and get less tense. Hope it helps!
Try to relax her. Turn a lot make big, small circles but piles out and go over them in different ways. Basically do unexpected things so she has to think about a lot. And of course do a lot of half holts and if she doesn’t listen just stop her completely