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I suggest that you call the police and ask to have her either charged with harassment or else at least warned to stop contacting you.
You can show them that there are persistent, unwelcome, threatening messages to you.
As well, it sounds like she has committed fraud with her insurance company because they will not reimburse her if you charge her for unused sessions. They don't even reimburse for a missed session when the person is sick or for a late charge when they show up too late and miss the session so it may be that you should send a list of when you did do the sessions to the insurance company if you feel that is something you wish to pursue.
As well, you can call your college (your regulating body) and seek some guidance. Explain what is happening and say you are afraid she will make false allegations about you.
Finally, I suggest you have a lawyer send her a letter threatening a lawsuit if she doesn't cease and desist at once.