When you say "she told the officers that I didn't hit her. And she is going to testify in my favor" I assume the "she" in that statement is your girlfriend and not the witness who originally called 911. If that's the case, the state can proceed with the witness even if your girlfriend says it never happened. That does not mean they will, just that they can.
You need a good criminal defense lawyer. If you don't have money, you can get a public defender. (Many of them are quite good. I was a PD for 9 years in L.A. and my colleagues were first rate.)
The fact that your girlfriend is saying you did not hit her is helpful, but the lawyer will need to deal with the witness. The lawyer will want to listen to the 911 tape and interview the witness to find out why she is making the claim (honest mistake or some other motive) and to develop a strategy to attack her version and/or persuade the prosecutor that the case should be dismissed. Naturally, the lawyer will want to speak with your girlfriend to get her version and prepare her for trial.
I hope this is helpful and wish you the best of luck. After you click Accept, I'll be happy to answer any other questions you might have.
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