How do I stop yawning so much? I already sleep 7 hr a night.

Not enough. I don't know how old you are, but if you're yawning a lot, and only getting 7 hours a night, you need more. There are a select few for whom 7 hours is enough, but most need at least 8 if not 10 hours a night. If you're falling asleep the moment your head hits the pillow, you are chronically sleep deprived.