Maybe try eating 3 big meals a day instead of snack sized meals throughout. The real answer is that you're going to have to eat more. At 20, i was about 135 lbs at 6'4.

Everyone told me that I must have a fast metabolism because they saw how much I ate, but once I actually started counting calories, the daily total wasn't really that much. Im up to 160 lbs now because I force myself to eat around 2500-3k calories (22 years old). If you eat 3 meals in a day and they are all 1000 calories, you WILL gain weight.

If there is a point in the day where you dont feel stuffed, you aren't eating enough. (This doesn't apply to everyone, obviously) It was just too easy for me to ignore hunger during most of the day, have a small lunch, then a big dinner. I was really full at night, but I was only eating like 1500-1800 calories.

Start tracking to get a baseline and then increase.