I’ve been eating too much lately, so my food expenses are high. To save money, I bring a rice ball for lunch—but when I get home, I still end up eating more. I'm gaining weight, and I want to stop as soon as possible.
Or for a more casual tone, you could say:
These days I eat too much, so my food costs are pretty high. I bring a rice ball for lunch to save money, but when I get home, I keep snacking. I'm gaining weight and really want to stop soon.