Keeping your shoulders up is tricky to get used to at first, but it is key! If you're approaching a fence and need more energy, leg & heels down is always the answer. Try very hard not to lean your shoulders, it can be a very bad habit when you start jumping bigger fences! :)
I have the same exact problem Maggie! Just really focus on pulling your shoulders back and keeping your heels down. You also have to learn how to keep your hip angle open😊 Hope this helped!😊
I had the same problem for a long time when I was younger and I still have problems with it occasionally now too. It really takes a while to overcome but I would recommend doing a lot of transitions practicing sitting up while having someone watch you.