It's been 14 days with an imediate upper denture, and I still can't keep it in my mouth for more than 30 minutes.

I hear and read about so many people not having any trouble with an upper, so it looks as though I'm the exception.
I'm at a total loss here! Why can't I get used to the thing?
It gags me so bad. I'm ok for over an hour if I don't open my mouth, but as soon as I try to talk, drink, or anything that requires me to open I start gagging. I'm now to the point where I don't even try and wear it.
Could the denture be to long? I did notice there is about a half an inch of molding behind the back teeth. Is that normal?
The Dentist has told me I would just have to get used to it, and the gagging would subside. Is that the only answer?
Sorry for the ranting, I'm just really upset about this.
Willy