Your divorce decree says your ex is responsible for the taxes. The IRS does not care. Joint and several liability under IRC Section 6013 means both spouses are on the hook for the entire balance, regardless of what any state court order says.
Your divorce decree gives you a cause of action against your ex for indemnification, but it does not bind the IRS. Innocent spouse relief under Section 6015 is the path to eliminating your personal liability. This is fixable.