English 'my' is the unstressed form of 'mine' (Old English 'min'), split in Middle English the same way 'a/an' split — 'mine' before vowels, 'my' before consonants — until 'my' generalized as the determiner and 'mine' became purely predicative.
Belonging to or associated with the speaker.
From Old English 'mīn' (my, mine), from Proto-Germanic *mīnaz (my, mine), from PIE *h₁mene- or *h₁mey-no- (of me, mine), a genitive/possessive formation from *h₁me- (me). The split between 'my' (unstressed, before nouns) and 'mine' (stressed, predicative) occurred in Middle English — originally 'mīn' served both functions. 'My' before consonants and 'mine' before vowels
'My' and 'mine' were originally the same word — Old English 'mīn.' Before consonants, the vowel shortened to 'mi/my' in unstressed speech. Before vowels, the full 'mine' was kept (Shakespeare's 'mine eyes,' 'mine honour'). Eventually 'my' won everywhere as a determiner and 'mine' was restricted to predicative use ('the book