It is not possible to recover all of your basis. As I understand it, in or before 2015 you rolled a 401(k) over to an IRA and that rollover included basis in after-tax contributions that had been made to the 401(k). A proportional amount of basis was distributed each year that you took a distribution even if you failed to report it correctly on your tax returns. Your basis is now whatever shows on line 14 of the 2020 Form 8606 after having correctly reconstructed the 2015 through 2020 Forms 8606. You can file amendments to your 2018 through 2020 tax returns to claim refunds of excess taxes paid, but 2015 though 2017 are closed tax years for which you can no longer claim a refund.
Incorrectly preparing your 2015 through 2017 tax returns does not mean that the basis that you failed to include on your tax returns for those years remained in your IRAs. You are not permitted to pick and choose the years in which you apply the basis; the law does not permit that.
