Offer's signed, I still haven't told my manager, and I've rehearsed the conversation about eleven times

Four years on this team. He hired me when there was almost nothing on my resume worth hiring, and he put me forward twice for things I clearly wasn't ready for.

Logically I know people leave and this is completely normal. It still feels like I'm about to let down the one person who took a chance on me.

Meeting's tomorrow at 11. Has anyone got a version of this that went well? I keep drafting something long and I suspect short is the right answer.