Most people who have had sex with more than one partner, have had a partner that did not measure up to their expectations. People will often give up on bad sex partners without giving them a chance to get better.
Is there a better way you can approach this?
What are your expectations from a lover? Has this conversation come up between the two of you? If she is not doing something, simply asking her to try it could be sufficient. If she is performing actions you generally like, incorrectly, try letting her know what you would prefer. Remember that not everyone is a porn star, and she may simply be doing what their last partner could not get enough of.
Breaking the news
How you break the news can really change the tone of the outcome. Let her know what it is about her you like, before moving on to what you would like to change. Take ownership for not having let her know sooner. Ask her if there is anything you could be doing better for her. Sex is a two way street, and like anyone else she will have preferences. If you cannot come up with more pros than cons, you may want to just stop seeing her.
The guidance that you provide her is as much for her benefit as it is yours. Praise what she does right more often than giving instructions. Like anyone, she wants to know that she is getting something right. Another way to incorporate direction is through dirty talk. If she enjoys it, dirty talk can be a much more fluid way of giving direction. For those women that are interested in getting better, but are too embarrassed to discuss it, there are many instructional guides available. You may wish to read the book first and highlight any passages you feel are poignant.
If the situation is unsalvageable
Even with expert instruction, not every woman will be able to meet everyone's expectations. What you may want her to do may be outside of her fetishes. Do your best to keep her feelings in mind when you break it off.
Telling a woman she is bad in bed is a difficult conversation. Give her the benefit of the doubt and let her know exactly what it is that you like. You may find that she has her own list of improvements for you. Facilitating this conversation will ensure that both parties are getting what they need.