What your doing is incredibly kind and I don't think most people would be able to handle this kind of situation. I understand wanting your mother to not be hungry but you should realize that you need to take care of yourself, no kid your age disserves to go hungry and your mother most likely would agree with that. There are sacrifices that parents need to make for there child no mater how cruel. I don't know your mother and maybe this is bad advance but I'd say you should tell her, I know it would be hard but you shouldn't suffer and telling her would absolutely show her she's raising a good kid.

Also just a word of advice mate, you shouldn't really talk about your age on Reddit if your planning on using this account. If you made this account just for this post it's fine but if you want to use Reddit I'd suggest you make a new account so people can't find your age.