When I was buying my daughter's horse, we physically looked at 13, rode them all, some a few times, and 3 of them we brought home for a 2-3 week lease to see what they were like in a new place. Then when we settled on one, I had the vet check done. I had heard horror stories just like yours, of the horses being drugged, or kept underfed so they were sluggish. As soon as they got weight they felt a little too good.
One of the horses we tried was ridable for 3 days, then she came up so lame that she could hardly walk...drugged.
The other horse that we tried was gorgeous, huge and marketed as a super mellow kids/husband's horse. When he was in the roundpen, my 10 year old daughter put a little pressure on him to change directions, and he jumped out of the 6 foot roundpen!
First question for you: What is your horse eating? Sometimes a change of diet can be partially responsible for a major attitude change. More than likely though, you bought a horse that was "under the influence" of something. I'm really sorry you got fooled.
Good for you for hiring an instructor. It's a hard decision, but don't be afraid to decide this isn't right for you either, especially since it's a horse for your daughter.