VERY long stride + VERY short stride

My Standardbred mare was bred to rack, but trained out of it. Her trotting stride is VERY long, and her cantering strides are soooo short!!! I've been told it's just her conformation, but I've also been told she can be trained out of this. SO two questions;
1-Should I allow her to keep her longer stride at the trot?
2- Is there anyway I can have her open up more at the canter, WITHOUT her just speeding up a lot? (her canter isn't that great either)