There are some important questions that you have not answered. Why did you not file a tax return for 2017, 2818, and 2019 previously? It would have been permissible if your income was less than $12,000, but that would be unusual unless you are retired.
It is important to make sure that the IRS has accurate information for all of your tax years. Because the IRS thinks that you filed joint returns for 2017, 2018, and 2019, the IRS could hold you liable for factual inaccuracies on those returns until you can prove that the returns were fraudulent.
I would start by filing a form 14039-T, identity theft affidavit. You will need to explain the facts and circumstances, and you may wish to include a copy of your divorce decree or at least the final signature pages with the judge’s signature and the effective date, to prove that you were single on or before December 31, 2017.
Next, you need to file your own accurate single tax returns for 2017, 2018, 2019, and 2020, even if you did not have enough taxable income to be required to pay taxes. It will be critical to get all of the facts on file correctly. If you didn’t file those years because you have been working under the table, now is as good a time as any to come clean and pay what you owe and make sure the problem doesn’t compound in the future if this fraud goes in corrected. In that case, you may want the advice of a tax professional because they may be able to help you negotiate a reduced settlement with the IRS.
There is an additional fraud reporting procedure, but it looks like this procedure does should not be used when the fraud involves yourself. In that case you stick with the 14039-T. But here is a link just in case. https://www.irs.gov/individuals/how-do-you-report-suspected-tax-fraud-activity
You will also want to visit ftc.gov for additional information on identity theft. There are a number of additional things you should be checking and steps you should be taking to make sure that your identity can’t be further abused by your ex. You will need to get copies of all three of your full credit reports and make sure that additional debt was not taken out in your name. You will want to freeze or lock your credit reports so that new debt can’t be taken out in your name. You will want to go to the IRS online account website and claim your online account before your ex gets the idea to claim it for herself fraudulently.
