How tall are you?
Are you sure you haven't got Celiac's disease-it's quite common?
100 pounds is light regardless of your height. Try planning your meals throughout the day. Eat snacks in between. Watch lots of movies, I always feel the urge to eat something whilst watching films.
It would be useful to know you're height to work out a BMI. I would see your GP as they might be able to give you some advice and send you for some tests to check that it's not some other health problem that's causing you to be underweight (as you say you struggle to put on weight). I think most people, even when they recognise that they're very skinny, are a bit nervous about putting on weight because it goes against the general ideas of society to lose weight. But don't let that deter you! Despite popular belief that putting on weight is "easy", it's difficult and does require determination to do it. You should make meal plans, work out how many calories you need to eat to gain weight, go for calorie dense foods that aren't overly filling - nuts are very good for this, as are some junk foods (but don't just binge on junk food - try to keep some healthy stuff too!). You've got to eat even if you feel full - that's how you put on weight.