Gaining

Hun, you are 18. Your metabolism is at it's peak right now. There's only so much you can do to overcome it. You can try eating more and moving less, but if that doesn't help, you may need to wait a few years for it to naturally slow down.

It is possible to gain weight with a fast metabolism. I've slowly been doing it over the last couple years it just takes a lot of patience and you have to fight for every lbs you gain. You just have to constantly be counting your calories and making sure you're eating enough.

Your metabolism will slow as you age. I'm 20 and I can already tell mine has slowed down some since I was 18 as I've been able to pack on some lbs. So just be patient and you'll see gains eventually.

Metabolism increases when you gain weight too btw it’s why gainers plateaue. The bigger you are the more calories you need to maintain your weight