Ouch. I am assuming that you did not report them as deductible but simply didn't report them at all. Several years is more than the 3 years back allowed for amending a tax form. You could at this point quickly amend for 2016 and 2017 and then 2018 by add a form 8606. However, the basis you can report wouldn't include prior years. Perhaps the most feasible solution is to keep track of your history of nondeductible contributions and keep evidence of them in terms of cancelled checks and account statements. The IRS says in Publication 590-A (www.irs.gov/pub/irs-pdf/p590a.pdf):
Failure to report nondeductible contributions. If you don't report nondeductible contributions, all of the contributions to your traditional IRA will be treated like deductible contributions when withdrawn. All distributions from your IRA will be taxed unless you can show, with satisfactory evidence, that nondeductible contributions were made.
