As someone who too often frets and gets anxious, I am very fond of this prayer by St. Francis de Sales.
Be at peace.
Do not look forward in fear to the changes and chances of this life;
Rather, look to them with full confidence that, as they arise,
God, to whom you belong will in His love enable you to profit by them.
He has guided you thus far in life,
and He will lead you safely through all trials;
and when you cannot stand it,
God will bury you in His arms.
Do not fear what may happen tomorrow;
the same everlasting Father who cares for you today
will take care of you then and everyday.
He will either shield you from suffering,
or will give you unfailing strength to bear it.
Be at peace, then, and put aside all
anxious thoughts and imaginations. Amen.