While you could have made him pay for half of the debt before he filed bankruptcy, you were always liable on 100 percent of the balance. That means the creditor has the right to demand full payment from you, even if your ex-husband had not filed bankruptcy. In that case, you would have been able to go after your ex to pay his half. That is easier said than done, of course.
It is a shame that you never addressed this 12 years ago. That balance has likely grown quite a bit in the past 12 years and you are probably liable on an entirely new balance. Even though the creditor can't collect from him, his name still remains on the case, so both names still appear on the lawsuit.
The probable reason you are still liable 12 years later is because, if the creditor got a judgment against you, essentially suing you for the balance, that judgment is good for 10 years. At the end of those 10 years, the creditor can file a request with the court to renew it for another 10 years. This renewal can happen over and over again. Not all creditors file for that request, but if that letter is a judgment against you, it would appear that your creditor did. That means the judgment is still valid.
At this point, you don't have many options. You can try to settle the debt for a one-time lump sum payment or call and arrange a payment plan. You may also be eligible to file bankruptcy yourself. Until you choose one of these three options, your wages and bank account are at risk. This creditor can levy bank accounts to pay the balance or garnish up to 25 percent of your take-home pay.
You have ignored this long enough. While I know you believe it is not fair that you have to pay back the entire balance, you can't concern yourself with fairness now. You must act soon.