You are not permitted to recharacterize the contribution for 2018 because it is now well after the due date of your 2018 tax return, including extensions. You can't make an excess contribution for 2018 be an excess contribution for 2019 because it was contributed for 2018, not for 2019. The only permissible ways to resolve an excess Roth IRA contribution made for 2018 after the due date of your 2018 tax return, including extensions, is to either obtain a regular distribution of the excess or to apply the excess as a subsequent year contribution that you are eligible to make.
Even if you were eligible to apply it as a contribution as a subsequent year's contribution, you are not then permitted to recharacterize that contribution. You are only permitted to recharacterize contributions made "during the taxable year." (Contributions made "during the taxable year" includes contributions made for the previous year between January 1 and April 15 of the following year. Yes, that's a bit awkward because another part of the tax code results in these contributions for the previous year being treated as having been made during the previous year.) That's the way the law is written:
§ 408A(d)(6) https://www.law.cornell.edu/uscode/text/26/408A
