Anyway, St. Thomas had a famous chastity-preservation story that ends with angels protecting him for life. There's a specific prayer for chastity, that's attributed to the Angelic Doctor, so-called because he observed chastity as if he were an angel. Here it is:
I know well that every perfect gift, and above all others that of chastity, depends upon the most powerful assistance of your providence, and that without you a creature can do nothing.
Therefore, I pray that you defend, with your grace, the gift of chastity and purity in my soul as well as my body. And if I have ever received through my senses any impression that could stain my chastity and purity, I ask you, who are the supreme Lord of all my powers, to take it from me, so that I may with a clean heart advance in your love and service, offering myself chaste all the days of my life on the most pure altar of your divinity.