One option you might employ is to pay the amount that you believe you legitimately owe and default on the balance. If your only income is SSDI, you may be "judgement proof." Even if your therapist were to sue you and win the suit, your income from Social Security could not be touched. So, unless you've got a bunch of money sitting in the bank, or some other large and liquid asset, most anyone you owe money to pretty much has to take whatever you decide to give them. (An exception to that would be the IRS.)

This might put your relationship with the therapist under a severe strain, but I don't think I'ld want to keep working with a therapist who I thought was ripping me off.

If I were you, I would send the therapist a written explanation of what I thought was wrong with the bill. You really should do this, if all you've had so far is a verbal conversation. Then I would send a check for the amount I believed I did really owe and let it go at that. You might very well not hear anymore from the therapist.