Hi, Welcome to Health care magic forum.
As you describe, you are very lean, and want to gain by any healthy means.
I advise you to take, more calories, fats, fries, and oily foods, try to have limited heavy meals.Avoid restrictions on foods, to have a balanced diet.
Increase your sleeping time a bit more, and try to sleep, during day time when ever it is possible.
I advise you to consult a physician for diagnosis and treatment .If you have genetic cause of the low weight you can get the weight with difficult, and if not you can get easily. Once you get the weight you cant go back, but can proceed further, you should be intelligent to stop it.
I usually prescribe to my patient with such symptoms injections of anabolic
steroids periodically,Multivitamins, calcium, and protein supplement of your choice.
Wishing for a quick and complete recovery. Thank you.