"O God, Refresh and gladden my spirit, purify my heart, illumine my powers. I lay all my affairs in Thy hand. Thou art my Guide and my Refuge. I will no longer be sorrowful and grieved. I will be a happy a joyful being. O God! I will no longer be full of anxiety nor will I let trouble harass me. I will not dwell on the unpleasant things of life.
O God! Thou art more friend to me than I am to myself. I dedicate myself to Thee, O Lord."

I like this because it's very calming and strengthening at the same time. It's one of the first Baha'i prayers I memorized.