Im one of those people that has never had a side effect from Byetta, (well almost never-had nausea from to much pizza). However, it your body isn't getting used to it but on the other hand it is getting progressively worse, you may be building an alergic reaction to it rather than a tolerance. Im not sure your doctors approach is going to help, but its worth a shot.
Im wishing you nausea free days!
Warren