Suggest you eat 5-6 small meals a day and eat regularly. That helps with digestion and you will be able to get more nutrients into your body. Suggest eating healthy choices such as the healthy heart diet. Heart Healthy diet which is a moderate fat diet and a moderate fat diet means = using polyunsaturated (omega 3, corn oil, nuts, etc) fats and monounsaturated (olive oil, canola oil, peanut oil, etc) fats, no saturated fats (high fat snack foods, butter, fat spreads) and no trans fats. Eat low fat meats (chicken, fish, loin of pork) or non-meat sources (soy products, nuts), low fat vegetarian combinations (rice & beans, pasta & beans, low fat cheese & pasta), low fat and substitute dairy products (1% milk, low fat/fat free yogurts, low fat soy milk/soy yogurt), use plant oils, non trans-fat margarines, and eat a lot of fruits and vegetables. Eat daily high fiber breads and cereals (oatmeal). Hoped this helped you.
You could add probiotic and/or digestive enzyme supplements to see if that helps. Also, I think you would be better off just trying to increase what you each meal little by little, by say a tablespoon at a time for a week or two until you are used to it, then add another tablespoon, and so on until your body is comfortable with it. Trying to force yourself to eat more than you are comfortable with and then feeling sick is not likely to work for you in the long run! My son is extremely underweight, and we are working on all kinds of options too! The supplements mentioned above are helping him.