Debt collector keeps raising my payments

by Robert
(Auburn, Alabama)

I live in Alabama. I have been making monthly payments every month to my creditor's lawyers. At first it was two hundred and seventy-five dollars a month. I was to contact them every six months to talk of an increase in my payment. The next time I called them, they upped my payment to three hundred and seventy-five dollars.
I am to call them again in August, and I am almost sure that they will want four hundred and seventy-five dollars then. I see no end in site for them increasing my payments every six months. The problem is that I am on a fixed income and I cannot keep increasing my payments to them. I am afraid of garnishment of my wages or some other form of harassment if I don't keep increasing my payments.
The amount I owe is large, around $24,000, because interest kept accruing during my protection from a debt reduction company.
How do I talk to the lawyers to get them to accept a payment that does not go up a hundred dollars every six months? Or is too much for my budget?