I've been having a problem with gaining weight since childhood and now it's getting more difficult to gain. i'm 21, 1.75 m and about 50 kg,please help?

Gaining weight: you've had a problem with since childhood and still you are, however you gained height and developed, your BMI is 16.3, should be > 18.5, so you need to gain between 7-10 kgs to reach the "ideal" weight,anyhow ideal weight is just a number,what matters more is the function,so long you are active,performing well, not getting ill and weighing 70% of your ideal weight,it could be just ur habitus