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5.1 with 45 kg is good, you can be said underweight
only for 2 or 3 kg!!! so i think you are not satisfied may be because of shape of your body.you can go gym because you need to do particular exercise for the shapes of your chest, waist, arms,thighs etc. gym have good facility for this type of people. i can assure you will have good personality because i experienced same thing with my own self too ! its the matter of 2 3 months only. plz concentrate on high protein diet, plenty of milk and its products, fruits esp bananas, khajoor etc.
I hope I answered your query. Wish you good health.