How old are you, if you don't mind me asking?
I ask because this was me about 15 years ago. No matter what I ate I could not gain weight. I was slightly underweight and always tried to gain, but nothing I did worked. My friends and I joked about it and I would eat the fattiest foods possible. It wouldn't matter.
Eventually my metabolism slowed and I gained weight and now I am like most other people in the US where I weigh more than I would like and have to work hard to lose the weight and keep it off.
If you are still young then it is possible that as you get older your metabolism will slow down and you will not have this issue anymore.