Hello,
Thanks for posting your query.
Please do not worry and do not let your appearance hamper your self esteem.
Most young females gain significant weight after marriage usually without any diet measures.
Still since your weight is definitely low for your age you should try the following measures.
1.Add butter or trans-fat free margarine (or ghee ) to daily food.
2.Use whole fat dairy products such as whole milk; and cheese, cottage cheese and yogurt made with whole milk instead of skimmed milk
3.Modify Cooking: Use heart healthy oils such as olive oil or canola oil, and add nuts when preparing and cooking food. Experiment by adding almonds, walnuts or cashews to salads or a porridge ( kheer) or cottage cheese ( paneer).Try all-natural peanut butter with sandwich as a wholesome snack.
4. Yes continue having bananas and milk and try to eat five meals a day , breakfast , mid morning snack, lunch , evening fruit or snack and dinner
5. If you feel despite these measures there is no significant weight gain then you should get yourself evaluated by your physician or a nutritionist and if needed protein and multivitamin supplements should be started.Most over the counter multivitamins available nowadays do have a good balance of all required nutrients and can be taken once daily.

Lastly there are weight gain supplements too ( ensure plus, boost plus etc) but its better to gain weight gradually and naturally around 1/4 to 1/2 a kg per week will be appropriate.
Once your fat intake and weight increase your breast size will increase too and in addition you can undergo muscle training exercises under a trainer for a fit upper body.
Hope that answers your query.
Wishing you good health.
Regards

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