When it comes to eating healthy, you don't always need to limit yourself. In fact, you really shouldn't limit yourself to just one food at meal times.
You should aim to eat three different types of foods at each meal, and not just any food but three nutritionally dense foods. You know, foods that provide the most nutrients with the least amount of calories.
I think this is a great way to eat but find this concept most difficult to follow at breakfast; I have been working on it. I have a whole wheat waffle with yogurt and half a grapefruit. I am eating three different foods and getting some fiber, calcium, protein and vitamin C. For lunch, similarly don't just have an all lettuce salad — add some protein and some cheese or a whole grain roll. Adding a variety of foods to each meal will help cover some gaps in your nutrition.