Lissylou-
Your lunch doesn't sound like enough food to me. I would have a piece of fruit (banana or applesauce or mango) and maybe some pasta salad w/ veggies mixed in to go with your sandwich. You'll probably feel a bit fuller by incorporating more fiber into your diet.
I'm curious to know what you are eating for breakfast? And if you are eating a snacks between meals, what are you choosing?