Starting Again

Hi everyone!
I'm a new member on here so forgive me if I mess up at all! I've recently started riding again after a two year break and I'm absolutely loving it but finding myself a bit stiff after my lessons. I've been riding since I was little and, not to blow my own trumpet, I was really good before I went to university. Now, although I'm progressing OK, I feel really disappointed that I've gone from being top of the class to the bottom. My main problem is my position in canter. My instructor tells me that I collapse on one side and I've actually noticed that this is true when I sit in a chair normally. Whenever I notice my self doing it I correct myself. When I'm cantering, however, this all goes out the window. My legs swing about and my seat just will not stay in the saddle! I really need your help. Does anyone have any tips or exercises I can try to keep my legs and seat where they are supposed to be?!
Look forward to your advice!
Lynz xx
 
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