well, What I did(Mine was the same way), is I thought of some rewards to give her when she does particularly well. These make her realize you are pleased, and most all subs/slaves live to please their Doms. they help to keep them striving to do good. Asfar as punishment, there are soo many options besides pain. Mine loves pain, but HATES to be bored. So recently I made her stand holding a coin against the wall with her nose for 2 hours. Trust me when I say it was very very effective. The key is to find something she does not enjoy for punishment.
Some subs require a firmer hand than others. It sounds like you need to dig deep inside and find the confidence you have to have to take control of the situation. If your trying to spank her on the ass, and she resists,and you strike her leg, she'll get the point quick enough. You just can't give in to her struggling. As long as she knows she can make you quit, she will keep it up.