I converted in Dec 2017 and I converted more than I should have?
Yes, you can do a recharacterization of your Roth IRA conversion to reverse all or part of your conversion.
Please read this TurboTax article for more information:
https://turbotax.intuit.com/tax-tips/investments-and-taxes/reversing-a-roth-ira-conversion/L0yVOfwyZ