I just paid Fed tax into 2018 not 2019. Will it roll into 2019?

What exactly do you mean by "paid Fed tax into 2018"?  What exactly did you do?  How did you pay? By what means?

using an on-line 1040 payment center, I paid what I thought I would owe for self-employment income.  I clicked the 2018 box, not the 2019 tax box.  My 2018 taxes are closed and paid - nothing owed

The payment will not "roll" into 2019. You now have an overpayment for 2018. Call the IRS at 800-829-1040 and ask them to either move it to 2019 or send you a refund of the mistaken overpayment.