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 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.