Ensure would be a good between meal supplement, one to three times per day. Sustacal, Meritene or even Instant Breakfast would also be appropriate.
Some exercise programs can suppress appetite and long distance runners can lose blood via the gastrointestinal tract. I am concerned that you do not eat meat which is a complete source of protein and red meats contain iron. Carbohydrates and fruits are a significant source of calories and vegetables a significant source of vitamins. However, you are missing meat and milk. The My Plate which is a good basic healthy diet recommends 2 to 3 servings of milk, 2 to 3 servings of meat, 2 to 4 servings of fruit, 3 to 5 servings of vegetables and 6 to 11 servings of bread, rice or pasta.
You may have to approach your diet from a training program approach. Eat 3 regular meals per day with a variety of foods chosen from the My Plate plan.