Stretching downward may be a desirable thing but only when the rider indicates its time for the horse to do it. NOT. When the horse decides he wants to put his head down and rip the reins out of your hand. Do NOT accept or allow that. It may be a evasion and my be due to him wanting to escape poor hands, on your part. But you do not allow that. When he does , you boot him forward. He'll put hs head up and move forward. After that , you can get someone to help you get your contact/hands to be better.
If you'd like, you can post a video here for a riding critique