Keep your heels down and try to work on strengthening your seat. Seat aids can be very nearly as effective for driving forward, changing direction, and stopping as legs can be. But you really need to try to slowly get your mare used to leg pressure. You want a horse to not be numb, but comfortable with things like that. For example, if she spooks and you lose your balance, you want to be able to use your legs to grip and steady yourself without creating a whole new problem, right? But really, experiment with your seat next time. Try shifting your weight around and see what she does. If she's sensitive to leg, she should respond well to seat aids.