Kate and William knew each other for a decade before they got engaged. They'd lived together, she'd been subjected to years of media pestering. She knew what media hysteria surrounded Diana. She had also watched decades of how Camilla rode the storm and survived.
Kate went into her marriage and the royal family with her eyes wide open, knowing the score, and she'd seen two polar opposite examples of how to handle it, from Diana and Camilla, She knew what to expect, what was expected of her, and what not to do.
Meghan didn't and still doesn't.