Once a horse is working evenly, correctly and confidently into a contact, with adequate impulsion, engagement and swing over the back, it will naturally wish to stretch long and low over the back. Achieving a 'pretty head' by jiggling the reins and them expecting the horse to travel long and low does not cut it. The horse may lower its head, but it will fall immediately onto the forehand and completely eliminate the whole point of long and low, which is to stretch and lift the back and neck.
Don't bother with flying changes until you learn how to ride the horse forward into the contact or you're horse will struggle - being on the forehand through a change will probably lead to your horse becoming disunited and very sour.
I suggest you find a good dressage instructor (not just a pony club instructor) and learn how to ride a horse forward and through, which will lead to long and low.