Dear Jarrod, 

Although prayer will no doubt help you strengthen your resolve, I know of no prayer or devotion that will of itself get your weight under control. That is something you have to take personal responsibility for. 

Some things may help: for example check with a doctor to see if your weight problem reflects some glandular or other underlying medical situation. If it does not, it is time to look at your life-style as people call it. How active are you? How much time do you spend lying around? What, when and how often do you eat? Do you suffer emotional stress that drives you compulsively to eat all the time? 

Generally speaking for a young person a little care in the type of food eaten, an increase in physical activity, a compelling reason to get results (in your case to be a better priest) and the ability to stick at it is enough to start making progress. From all I've heard it is much better to take the weight off slowly but surely rather than a crash diet. I know a college student who just by walking a couple of miles to and from classes each day made the biggest and long-lasting difference ever in his weight, over the course of two semesters. 

All the best.