hi lonely devil,
after looking at how you eat, you should be overweight by now...
it make sense if you are eating that amount of calories, and your thin body just can burn around 1300 cal, the balance cal should be keep and store in your body as fat.
if by now you go and do medical check up on your visceral fat and body fat, you are having extra internal fat that can lead you to serious diseases and illnesses even you are thin.
THE problem in your body is your absorption system, which is your intestine.
you eat alot, but not well absorb.
you are young, if you have time and money to risk your health, you can keep eating...
lots of thing in your life need to change, if you want to improve your health.
anyway, let me know if you want more details on how to improve your absorption system first.
good luck