When a person is charged with a felony, the question should never be what you will be telling the prosecutor. An attorney is to be an intermediary between you and the prosecutor and the police. The advantage of an attorney is not only the fact that he is trained to be your advocate. In addition, your attorney will be able to discuss your defense without having your own words be used against you in court. Your attorney will know what to explain and what to leave unexplained. If the case cannot be resolved, your attorney will be trained to present your case in court. There is too much to lose when charged with a felony to even consider handling the case yourself.
Why would you say anything to the prosecutor or the police? They are NOT there to help you, and they will eagerly find any way to use your statements against you. The only person you should be talking to is a criminal defense attorney.
You need a lawyer to help you with this. Talking to the prosecutor will not help you any more than talking to the police would have. This is a serious situation and not one you can handle on your own.
This response is for informational purposes only and is not offered as legal advice.
Please, please, please get an attorney. There is a decent chance that an attorney can clear this up and probably keep the girlfriend out of trouble, but you do not want to be trying to do this on your own (even discussing this issue with the girlfriend could get an over zealous officer to charge you with witness tampering). Especially if this could come up with probation, you need to get an attorney on this now.