Where does “我が” come from?

我が (Japanese) comes from Japanese 我, from Old Japanese 我, from Old Japanese れ.

我が (Japanese): my; our

Definitions

Step	Language	Word	Meaning
1	Japanese	我	first person singular pronoun: I, me, oneself;...
2	Old Japanese	我	first-person personal pronoun: I, me;...
3	Old Japanese	れ	—

我が — Etymology, Origin & Word History | Etymology Explorer