What an interesting question!
Although I don't think that's so simple…
Myself used to be meself actually.
Most forms had the dative (as in himself), there was also a difference between her (dative) and her (genitive), and the first one was gradually changed into the second one. The same thing happend with meself > myself.
This might not be 100% accurate so I'd suggest checking it in a trustworthy etimological dictionary.
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