My first on-call rotation starts next month and I'm quietly dreading the 3am page

I'm about six months in and just got added to the on-call rotation. Logically I know the point is to learn the system, there's a runbook, and a senior is on backup. Emotionally I'm bracing for the one night something breaks that isn't in the runbook and I'm the person expected to fix it half-asleep.

For those who've done it a while: what actually helped the fear settle? Did you shadow someone first, keep your own notes, something else? Trying to walk in prepared instead of just anxious.