I disagree. Yes, prophesy can be self-fulfilling but Zelena has not been defeated if her power lived on in her pendant. She still found a way to change the past by opening a time portal. What she didn’t predict was Emma going back in time to defeat Past Zelena once and for all.
Just like we all expected Rumple’s undoing to be him loosing his status as Dark One, we all expected Zelena’s defeat to be loosing her powers.
But I don’t think it was. Zelena’s defeat was loosing that last bit of innocence and changing fully into the Wicked Witch. Hence why her necklace changed from white to green.
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"He was a lot of things to me" "The only conclusion was love"