Ugh, that's a hard situation.
I'm presuming you're younger so moving out is not an option. The good news is you can still eat *less* even if you can't eat differently, and you can still workout. Is there any possibility of changing up your breakfasts and lunches--say, if you pack a lunch for school? That would at least help the nutrition a little bit. I don't know your family situation, but if either you have your own money or your parents would be willing, it doesn't take much money to buy things like eggs and whole wheat bread and apples and such.
As to the motivation, hang out here! It's a pretty motivating group of people. Whenever I'm struggling with discouragement (like right now!) I just pop on here and read, read, read.