You need to get an attorney, and possibly also talk to a bankruptcy attorney. It sounds to me like you are not being treated properly here.
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An attorney cannot really answer your question without reviewing all the documents. Did your ex agree to "protect and indemnify you" in the event he defaulted on any payments he was supposed to make as part of the marital settlement agreement? Alston & Baker provides free consultations. If you are interested please contact us.
Please note that THESE COMMENTS ARE NOT INTENDED AS LEGAL ADVISE and are for informational purposes only. This response does not create any attorney-client relationship and is based on limited information. A full review of additional facts might change the answer, therefore you should not rely on this answer as legal advise. I recommend that you consult with an attorney who can review your case in detail, understand the specifics of your claims, and protect your interests before you take any actions.