What if I realize I thought I filed electronically, but I didn't. And I didn't mail in my tax return until after the deadline? What do I do now?

Since a 2018 tax return can be filed anytime after January 1, 2019, there is really nothing to concern yourself about.  The April 15, 2019 due date is if you have taxes owed so you need to file the 2018 tax return on or before that date to avoid any failure to pay penalties.

If you filed a 2018 tax return for a tax refund or there were no taxes owed, then there are no penalties for filing after the due date.

If you filed a 2018 tax return after April 15, 2019, had taxes owed and paid those taxes then the IRS will send you a bill in the mail for any penalties and interest owed.