I think dropping your stirrups will help with your posting and balance. Remember, don't grip too much with the back of the calf, but the side. Be careful not to grip with your knee! Also, when she begins to trot do not lean forward. This puts more weight on the forehand, throwing you and her off balance and encouraging her to walk to regain her balance. If I was riding a horse that was only giving me that much trot, I would have given her a hard kick and if that didn't work I would start carrying a crop!
Intermediate Rider~Learning Equitation and English Pleasure~Interested in Eventing
A horse will tell you everything you need to know about him, if you listen.