Wait, she wants you to pay for sessions that you had canceled in advance? That doesn't seem fair. The only possible thing is I've heard of some people having an arrangement where they have a set time slot and have to pay whether they come or not (even if they know, say, weeks ahead they'll have to miss due to a vacation, say). If you had that sort of arrangement with her, then you may not have an option. But I imagine you would have had to sign something agreeing to that.