“Thou'rt yet to become me”

“O'Radagon, leal hound of the Golden Order, thou'rt yet to become me. Thou'rt yet to become a god. Let us be shattered, both. Mine other self."

The more I think about it, the more this quote only makes sense if you understand Marika as being in a struggle with Radagon.

It sounds as though Radagon is attempting to take over Marika's body and will (possibly at the behest of the Golden Order, sensing Marika’s waning faith and impending betrayal) to 'become a god' (quite literally becoming Marika), and that Marika is resisting this by shattering the Elden Ring (i.e. the source of her godhood), after realizing Radagon's play but before their merger could be entirely complete.

This explains why Radagon has imprisoned Marika/himself, and is hanging around desperately, in vain, trying to fix the Elden Ring - but he can’t because he’s not a god. Marika is a god (her being female seems to have something to do with it too).

Marika is Radagon, but they are still two different wills.