That sounds horrible, what happened to you. And I must say, a VERY irrisponsable instructor!
In the long run, it would be best to go back to the barn that you KNOW is going to look after you, and that you KNOW has horses which are more suited to your level. $15 isn't that expensive, I'm paying $35 for a group lesson, and $45 for an individual. Talk to your parents, save up those $$$$ and spend your money in a place which is worthwhile and is going to ultimately nurture your riding skills/love of horses, rather than waste $15 on a lesson that's going to a) hurt your confidence and b) be potentially dangerous, and not just to you, either! It's dangerous for anyone to be placed on a horse which is outside their skill and confidence level, no matter how long they've been riding.
The art of keeping a horse between YOU and the GROUND!