Reinhold Niebuhr wrote the first known version of the Serenity Prayer, and it's been through a few revisions just by popular word of mouth or even literal Hallmark greeting cards, but the version they say at AA meetings is:

God, grant me the serenity to accept the things I cannot change, the courage to change the things I can, and the wisdom to know the difference.

The basic principle can be decoupled from the religious origins and still be a useful approach to life. Learn not to stress about the things not in your control, while still taking control of the influence you do have.