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@JenniferEdward 

I recommend that you talk to a professional tax accountant, preferably a “enrolled agent“; this is an accountant who especially admitted to handle IRS matters.  I will try to run over some of the basics for you, but your situation is too complicated to fully handle in an Internet discussion forum.

If you are legally married, you were required to file as married – either married filing separately or married filing jointly. As mentioned, joint filing is always allowed and it is usually preferable.  If you were permanently separated, and you provided care in your home for a qualifying dependent, you may be able to file as head of household. The IRS usually considers medical separations to be temporary and don’t qualify you to file as head of household, however, it might be reasonable to treat the separation as permanent if you knew that your spouse‘s medical condition would never allow you to live together again. That would be a matter for expert review. 

If you filed single, that is not allowed. If you filed head of household, that might be allowed if you provided care in your home for a qualifying person.

However, now we turn to your spouse. Because your spouse was married, your spouse must file as married, either jointly or separately. If you filed single or head of household, your spouse would have been required to file as married filing separately.  Then, your spouse‘s SSDI would have been taxable income. SSDI may or may not be taxable on a joint return depending on the amount of the other spouse‘s income. But when filing as married filing separately, SSDI is automatically taxable.  Over the period of time you were talking about, the standard deduction started at around $6000 and was raised to $12,000 for persons married filing separately. That means that if the SSDI was more than $6000 per year, it would have been partly taxable because your spouse should have filed as married filing separately. Likely the IRS does not know that your spouse was ever married, and since SSDI was their only income, the IRS assumed that the income was non-taxable, a tax return was not required, so they never sent an inquiry.

Lastly, let me talk about the statute of limitations. The IRS can audit you for three years in the case of simple errors, and they can audit you for up to six years in the case of an error that resulted in an underpayment of tax of more than 25%.  The clock on the statute of limitations only starts when a tax return is  actually filed. For someone who never filed a tax return, the statute of limitations never started to run so it has not run out, In the IRS can I did that person indefinitely.

If you filed as head of household for the past years, and if you were legally qualified to do so (because you provided care in your home for a qualifying dependent and because your medical separation from your spouse was not considered “temporary”) then your returns were technically correct.  And,  as long as you never signed a joint return with your spouse, you are not responsible for your spouse‘s failure to file or pay taxes, if any were owed. You would simply move forward by filing as married filing separately for 2022 and single from then on, or as head of household if you are still providing care in your home for a qualifying person.  (“Widow“ is not a tax filing status. There is a status called “qualifying widow or widower“ which gives you some tax advantages for the first two years, but only if you are still providing care in your home for a qualifying dependent under age of 19. If you don’t have a qualifying dependent, you would file as single next year.)

If you do not have a qualifying person for HOH and have been filing single, then your tax returns have been incorrect for those years.  The IRS could potentially go back up to six years and audit  you.  You would fix this by filing amended tax returns using married filing jointly and including your spouse‘s information, or by filing amended returns as married filing separately.  If you file amended returns as married filing separately, you would probably owe some additional tax payments, and maybe penalties and interest (although you can apply for a waiver of the penalties).  If you filed amended returns as married filing jointly, you might be entitled to an additional refund on the most recent three years of amended returns. However as I alluded to before, if you file a joint return, you are taking joint responsibility for your spouse‘s taxes and finances, so there might be a legal advantage to filing your amended returns as a married filing separately.  

If you do need to file amended tax returns, I would probably not go back more than three years, because you are probably protected by the statute of limitations.  However, my comment should only be taken to inform you of the generalities of the situation, and I suggest that you seek out the services of an enrolled agent.