I'm licensed in CA only, and you should always check with local counsel. Having said that, you can try contacting the District Attorney's office where the charges are pending, and sign a request to drop charges/no prosecution form.
Generally contacting the district attorney handling the case and informing them you no longer wish to participate in the case and then sending the defense attorney a notarized letter stating the same will be enough to get the case dropped.
A DA may not want to drop assault charges in many situations such as domestic abuse. The letter suggested by the other attorneys would certainly be considered. However, in many assault cases the DA does not need the testimony of the victim. For example, if the police arrived to break up the assault or the defendant admitted the assault and the police observed the injuries, there may be enough for the DA to prosecute. Good luck.
THESE COMMENTS ARE NOT LEGAL ADVICE. They are provided for informational purposes only. Actual legal advice can only be provided after consultation by an attorney licensed in your jurisdiction. Answering this question does not create an attorney-client relationship or otherwise require further consultation.