Sounds like your leg needs some reinforcement. Anebel posted some advice to get a horse that's generally slow to move out.
Originally Posted by ~*~anebel~*~
What I think is going to benefit you the most is shortening your reins a good 8" each, and then pressing your hands forwards and towards the withers. On a horse who is so behind the leg and tends towards on the forehand, the hand needs to be extremely passive, pressed forward and never used in a backwards fashion.
From there, really ride the horse forward into a working trot. If you are sitting, your abs should be burning. If you are rising it should be at a far quicker pace than what you are accustomed to. The trot you are riding now resembles more of a jog, and with the length of rein, how much you fuss with your hands and how little leg you are using, the horse is falling further and further behind you. You can see, every time the horse tosses her head, your elbows come out and back and then you are pulling. That is riding front to back. Back to front riding dictates that is the point at which the hands soften forward and you kick the horse forward.
What I like to do is tickle with a whip just behind my leg in tandem with my leg aids. Really get them to move forward, before you refine them.