You have a few options to handle an excess contribution for 2018. One way as you mention is to re-characterize to a Traditional IRA. If you want to keep the Roth IRA with the allowable contribution given your AGI (if this is the case) you may...
Request an extension to your 2018 taxes and make a withdrawal of the excess amount by October 15, 2019. A withdrawal is not a distribution. When you withdraw the excess contribution you need to withdrawal any interest that contribution has earned. Contact your Roth IRA plan administrator for help.
If you have already filed your 2018 taxes, there is a rule that allows you 6 months from the original due date of the return (October 15, 2019) to withdraw an excess contribution (even if you didn't request an extension). You may need to final an amended federal return and possibly state return once you have made the withdrawal. Again, your Roth IRA plan administrator should be able to help you.
As you note, It's important to fix this otherwise you may be subject to a 6% excise tax as long as the excess contribution is in the account.