Myself

google

ref

Old English me self, from me1 + self (used adjectivally); the change of me to my occurred in Middle English.

wiktionary

ref

From Middle English myself, meself, me-self, me sylf, from Old English mē self, mē seolf(“myself”), equivalent to me (pronoun) + self (pronoun), later partly reinterpreted as my + self (noun), my +‎ -self. Compare Scots mysel, mysell(“myself”), West Frisian mysels(“myself”), Dutch mijzelf(“myself”), German mich selbst, mir selbst(“myself”), Norwegian Bokmål meg selv(“myself”).

etymonline

ref

myself (pron.)

emphatic or reflexive form of I or me, c. 1500, mi-self, alteration of meself (c. 1200), from Old English phrase (ic) me self, where me is "a kind of ethical dative" [OED]. See my + self. The alteration from meself is by analogy of herself, where her- was felt as genitive (though analogous hisself remains bad form).