The problem is that you took the high road. Given what happened, you should have made an immediate complaint
to the police and had her charged. Unfortunately, you saw the situation as resolved and went on with your life. She took advantage of that by getting charges pressed against you instead. She can do that because all the police need to make an arrest and all the prosecutor needs to press charges is probable cause
. Probable cause requires very little evidence. It is just a reasonable belief that you may have committed a criminal
offense, and the word of the victim (even if the victim is just a good liar) can be enough to start the ball rolling.
The state needs a great deal more to convict you of this offense than what they have now. You must be acquitted if the state can't prove these charges against you beyond a reasonable doubt. So you do need a criminal lawyer. If you don't know where to start you could get a reliable referral from the Massachusetts Bar Association's Lawyer referral service. Their fee is reasonable, (around $50) and they promise to hook you up with an active member of the criminal bar in good professional standing. The referral fee includes a half hour consultation with the lawyer.
Talk to a lawyer before pressing charges. Now that she's already got a case against you, it is not always possible to get the same matter pressed against her. You are already a criminal defendant and should not be talking about this incident to the police and prosecutor since it can be used against you. Best to let your lawyer do the talking because he could do so in a way that wouldn't hurt your case. As for filing a charge for slander, that's a civil matter. And you would have to win this case first in order to claim that she said false things about you. Any loss or plea would make the falsehoods by definition true.