The Serenity Prayer is a good prayer to use while fasting. The Prayer of Saint Francis and the Prayer of St. Richard are two other prayers to use during a fast.
The Serenity Prayer is a simple, threefold request to God. It asks for the serenity to accept the things that cannot be changed, the strength to change the things that can be changed and the wisdom to know the difference.
The Prayer of Saint Francis is a bit longer. It is attributed to one of the most beloved Catholic saints, St. Francis of Assisi. It asks for assistance in spreading peace, love, forgiveness, faith, hope, light and joy. It also asks for help in being selfless. The prayer asks God for help in being more loving, understanding, giving and forgiving. The end of the prayer suggests that through the fulfillment of its requests, it is possible to be reborn.
The Prayer of St. Richard is by another Catholic saint, Saint Richard of Chichester. This prayer expresses the hope to love God more dearly, know God more clearly, and follow God more nearly each day.
These prayers encourage right action even when it is difficult. They can serve as sources of strength and inspiration during a fast.