I'm starting to think more in line with one repondants (thanks for that link!) thinking it's really just a variant of Mertis (that's how grandma would call my mom) so that would point to "
Myrtle":
Myrtle meaning and name origin
Myrtle \m(y)-rt-le, myr-tle\ as a girl's name is pronounced MER-terl. It is of Latin origin. Botanical and nature name based on the evergreen shrub that was sacred to
Venus as a symbol of love. First used in the 1850s.
Myrtle has 9 variant forms: Mertice, Mertis, Mertle, Mirtie, Myrta, Myrtia, Myrtice,
Myrtie and Myrtis.