The difficulty of GAINING weight: discussion

[SeeYouSoon]

What are the possible reasons why I'm having so hard of a time gaining weight? What are some ways to help? Right now I'm trying to eat every few hours or at least drink something like kool-aide to keep the calories pumping in. However, I just can't seem to hold on to the weight I put on.

A little about me: I'm 19, 5'6, 112 lbs, and I'm hoping to gain at least 10-15 lbs. (Is that reasonable/good?) I go to school full time and work a part time job on campus (4hrs a day) so I have the time to do what's necessary for me to gain weight and keep it. Any suggesstions? I do a lot of walking to and from school because my apartment is just off campus. Is that a bad thing and something I should stop doing? Should I consult a physician?

Sorry for the unorganization of my post, it's just driving me crazy that I can't seem to gain any weight these days.