How to slow down my horse?

The best way to get her to respond is not by pulling on the reins but in your seat and through half halts. You said you are making to keep your leg off of her which is good but could also be resulting in you relaxing to the point of your leg aids being useless. Squeeze with your knees to bring her shoulder blades together and force her to slow down. Also, just try to tire her out. She wants to go faster, right? So let her. Canter her (assuming she is in good shape) for a while until she is tired enough that she wants to slow down to the trot. The first couple of times she does, let her but if she gets speedy then push her back up to canter again. This will take the "fun" out of going faster. Eventually she will just learn to keep a good pace the first time.