If it's clearly one side more than the other, than there likely a reason. The very first thing you should be looking at is if it's actually you. Do you have a weakness through left that has allowed this to become a habit, or is it something you bought her with?
After that, have a look at the horse's saddle, teeth, hocks and back. An ill fitting saddle or an injury/pain can cause the horse to become very one sided (majority of horses are one sided anyway).
If anything physical is ruled out, then it's just a habit. If there's a dressage instructor in your area, I would recommend talking to them. If not, it's time and patience. How responsive is she to your leg? Can you do lateral work with her? Do you carry a schooling stick?