Epictetus: some things are up to us, some aren't. Spend your energy only on the first kind.
Epictetus (the label 'dichotomy of control' is modern) was born a slave and became a famous Stoic teacher. His handbook opens: some things are up to us (our judgments, choices, effort) and some are not (weather, other people, the past). Focus only on the first.
You can't control a Texas summer. You can control whether you water the yard at 6 am.
We can't control whether a customer replies. We control how good today's outreach is.
Practice it in Skepsis