That the peace of our heart does not depend on another's will, but lies in our own control.
The chief part then of our improvement and peace of mind must not be made to depend on another's will, which cannot possibly be subject to our authority, but it lies rather in our own control. And so the fact that we are not angry ought not to result from another's perfection, but from our own virtue, which is acquired, not by somebody else's patience, but by our own long-suffering.
Source: Institutes (New Advent)