I am puzzled by you saying your trainer says you are why your horse is stopping, but has the trainer not told you what you are doing AND HOW TO FIX IT? If not, you may need another trainer. If you are not (a) confident enough or (b) balanced enough to stay out of his mouth you will ruin him. That is a promise. I second going back to flat work, get that perfect, then start ground poles and work you way up. Forget the height right now, and get your timing and seat perfect, as well as give yourselves time to get confident and build trust TOGETHER. We all occasionally get off balance or get left, etc and catch our horses. SOunds like you may be doing it more "occasionally" than you should.