If you don't withdraw the money before April 15, you pay regular income tax PLUS a 6% penalty for 2018. Then in 2019, if you under-contribute, so that the 2018 excess is considered to be part of your 2019 limit, you pay no further penalties. If you contributed the regular limit in 2019, and still had this $2875 excess hanging around, you continue to pay the 6% penalty again and again. This situation would be covered on your 2019 tax return, you don't report your future plans on a past return.
If you withdraw the excess, it is still subject to regular income tax but not the 6% penalty. You need to make the withdrawal before April 15, and it will be a special procedure with the bank, not a normal withdrawal. Tell them you need to make a return of excess contributions. They will return the excess plus any interest it has earned, and you pay tax on the returned contribution and the interest.
