So what options are available for me to rectify the mistake (I forgot to include my self-employment taxes on my initial return)? Apparently, the IRS will go ahead and credit $10,000 (which doesn't really exist) to my 2021 estimated tax payments. In turn, I will need to pay them the $10,000 I owe them for my 2020 taxes due.
Instead of writing a check for the whole amount, what if I simply go ahead and make my estimated tax payments for 2021 as if the $10,000 "credit" actually doesn't exist? By the end of 2021, would everything be corrected? In order to avoid penalties, would it be better to apply for an extension for the payment due for 2020? Are there other options of which I am not aware? Obviously, I wish I had caught the initial mistake before filing. But I didn't and it appears that it is irreversible.
