I understand your concern. You can have calorie rich n nutritious foods like, banana smoothies, milk shakes with berries n fruits, fruit custard, nuts n dry fruits along with normal intake of regular meals, so do not skip meals n try to have small n frequent meals. Have a heavy dinner filled with roti, veggies n dal, if non veg can take lean meat or eggs. This will help you to gain weight.
It's better to go for natural ways.. try to eat skinned out boiled chicken 1-2 pieces twice a week, fish curry not fish fry as it is rich in vitamins, eat fresh raw sprouts everyday twice, include berries and fresh fruits like avocado,pears, apples, citrus fruits all this things are rich in fibres and vitamins.... Your morning diet should include whole egg and a glass of milk, try to repeat this in the evening also....include green leafy vegetables in your diet. Milk products like chesse,panner etc and meat can help you gain weight but since they contribute more towards fat and based on the history you have given, you need proteins instead of fat to build your body mass so avoid it. Drink plenty of water and stay hydrated. Do daily brisk walking, cycling for 5-6kms,yoga and meditation....if possible go for a trekking or join some rock climbing classes if such a facility is available in your place and do it on a regular basis which will help you in developing stamina....Consult dietitian they will do the needful.