The first thing you have to do is "rev" her engine. In other words get the hind end working and stepping underneath the body. Now she may be "fast" but still "out behind". How I would test this is if I ask for a canter from the walk - if I get ANY trot steps horse is not in front of my leg. Bring horse back to walk and (in time with horse's hind leg coming forward) rider should bump with the leg on that side. In other words - when your left hip come forward "bump" horse with riders left leg. A bump is NOT a hold rather hit/stop hitting with the isnide of the calf. Same thing for the other side.
If horse trots slowly bring it back to the walk. Do not punish for trotting (you did ask for more forward) but nicely correct to remain in the walk.
On the other hand - if you repeat this constantly then horse will start to ignore you - so bump hard if you need to, always firm so no question it was intended, but do NOT nag the horse constantly with your legs because after a while it will no longer work.
For the trot use posting to slow the horse down - try not to use the reins too much.