Welcome to Healing Well! I'm glad you were able to gain some insight by reading the posts here. You're definitely in the right place.
What did your GI doc tell you regarding medication? Unfortunately, 3 weeks isn't enough time to heal an inflammed stomach and esophagus. Your OTC Prilosec could be working just fine, but the tissue takes time to heal. 40mg is a dosage that many of us have had to use. It is true that some PPIs work better than others for certain individuals, but I'm surprised your GI doc didn't give you some guidance there.
You could call your GI doc's office and see if they think it would make sense to switch PPIs or if they think it's too soon to determine that it isn't working.
Are you sticking to a GERD diet to keep from irritating your upper GI further?
I would assume that your GI doc went over things you should avoid, and behavior that can help improve your condition. Here's one of may URLs out there--I'm sure you've read them already!
Have you elevated the head of your bed?
It would be very helpful in protecting your esophagus from further burning.
Hang in there. With time things will get better.
Again, glad you've joined us!