Try jumping without stirrups. This way to keep from tipping over, your leg will have to stay in the correct position. I also had this problem of kind of ducking when I went over jumps. If you have struggled with that stuff, then just purposefully sit up when you're going over jumps. I hope this was helpful!!
I think of sitting back with my rear and pushing my feet forward (they don’t actually go forward in front of the girth, just the mindset). This usually happens to me if I jump ahead of the horse instead of waiting for them to jump up to me.