M.B.
I can't really understand exactly what you're saying or what your diet is. However, you'll want to eat calorie-dense foods and foods with healthy fats.
Try dried fruit and nuts as a snack, or guacamole on chips or peanut butter on crackers. Try drinking whole milk instead of low-fat. If you can afford it, drink something like Ensure or Carnation Instant Breakfast between meals.