What will make me thinner?

I used to swim a lot. like a LOT. I was really, REALLY fast and it kept me in good shape. I developed a swimmers diet meaning that I would eat and eat and EAT and would never gain weight. but I've recently stopped swimming due to the fact that I have tons of homework and school is just getting in the way. I'm really scared that I'm going to gain weight.. so please help. I've read about bulimia and anorexia and that scares me. so I need something to make me thinner. I cannot stop thinking about food. I'm hungry but I've eaten a ton!!
HELP!!!
please.

you can start running to keep in shape or just do swimming in your free time hope I am some help