Проверьте на ошибки в грамматике пожалуйста:
1) I'm underweight.
It might be a good idea to add more food with protein in your diet.
That's not a bad idea.

2) Lack of concentration disturbs me in study.
You might feel better, if you go to bed earlier and start to do physical exercises.
Thanks. I will try it.

1) I'm underweight. -  It might be a good idea for you to add more food with protein to your diet. - That's not a bad idea.
2) The lack of concentration during my studies concerns me. - You might feel better, if you go to bed earlier and start doing physical exercises. - Thanks. I will try it.