I understand that you are struggling with gaining weight. It is not possible for me to really know what the cause of this is without doing a face to face interview. You may not be eating enough calories even though you are eating multiple times a day. There may also be a physiological reason, although it does not seem that your doctor has offered one. You may want to see a nutritionist who can look at what you're eating and determine whether you are eating enough to gain weight. You may want to also discuss with the nutritionist how much you are exercising and any other behaviors (or medications) you are engaging in that might be preventing you from gaining weight. A therapist might be helpful if you find that you are struggling to eat as much as is recommended.