Hang in there. It could be your metabolism. I was very small, weighed about the same as you and was about your height until my junior year of high school. I was actually nicknamed Olive (for the skinny girl in popeye). It really made me so very depressed because tight jeans were the "thing" back then, too. I am 33 years old. There were only one type of jeans that fit me and I couldn't even fit into the cool "Guess" jeans that were very popular.
During my junior year, I became more of an average size but was still considered small. I was a size four at that time. I finally hit 100 pounds and was happy.
I am bigger now but still have a fast metabolism. What once was my curse is now a blessing - in only some ways, I really need more muscle. I don't consider myself too skinny, yet. I have to be careful and really watch what I eat. If I don't eat a well-balanced diet, I will lose weight. I weighed 100 pounds before I got pregnant with my second child and I was too skinny. I lost a lot of my muscle mass even before pregnancy because I forgot to work out and eat right.
I have to eat more protein than the average person to keep weight on and build and/or maintain muscle. If I just eat carbs, the weight just comes off of me because I have an extremely low muscle mass as it is. I just had a baby and weigh 115 pounds, but need to start working out again to build muscle. I
That is my advice for you. Start working out on weights to build some muscle. My guess is that you are thin and have little muscle tone. Eat some healthy fats (not that junk food stuff ) like Avocado. Eat a lot of protein like nuts, fish and meat. Concentrate on avoiding high processed food and sugars. Don't work out unless you have been getting protein. If you have little muscle mass, you may burn more calories and lose more weight.
Good luck. I understand. I have been there.