Loose loose situation

I really don't know what to do with this client. They're owing me more and more now and its getting harder and harder for them to pay me. First they came up with a plan to pay me every week but after a couple of week they missed a few payments. Now they've cut their repayments in half. At the rate they're going it'll take forever for them to pay me off fully, if ever. Maybe I should think about suing them for all the unpaid invoices. But if I do that then I must be prepared to loose them as a long time client and go to court as well. And I might not get all the money back. Its a loose loose situation.