It will take time but you have to be assertive and let her know you are the boss. She now knows that if she acts up you will stop, so you can't stop, let her act up. Eventually she will see that she is not in control. My suggestion would be to have these training sessions on days you don't even really need to cut her hair so it won't be as frustrating to you. As always praise her like crazy when she does something good.
It will get better, just give her some time.
