I've been there before, being too skinny for my liking and wanting to gain weight.
The feeling is even worse when you're a guy, and you got teased about it in gym/physical education class for being skinny, when you surrounded by all these brain dead "jock" types that naturally had the genes for increasing muscle mass.
I spent my teen years thinking I'd never be anything other than a lanky, skinny guy, or in the extreme opposite my metabolism would slow down and I'd become obese.
The truth was, it doesn't/didn't need to be either.
I'm 184cm tall and at March this year before I started doing weights and eating heaps, I weighed 69 kilograms (152 pounds) and I was of course, fairly skinny.
Now, I weigh 80 kilograms (176 pounds) thanks to a weight training regime and a really good diet.
I'm not huge by any means, but I'm way less skinny than all the skinny guys/girls I know and for the first time in my life, I can look at my body in the mirror and not feel totally repulsed and disgusting. I actually feel good about my body now.
The bad part (but it's also a positive) is that it takes A LOT of discipline to gain weight like that and make good progress.
Binge drinking is a thing of the past for me (In fact, I haven't drank any alcohol at all for 4 and a half weeks now), I basically have to ensure I'm home everyday so I can eat well, which means I've had to say no to a lot of party invites that would have involved me staying over due to not being able to get home and I've gotta train hard and smart with my weights program.
Yeah, I may have missed out on a lot of fun, drunken nights, making new friends, but looking at my progress in the mirror and on the weight scales, it's been totally worth the sacrifice and hard work.
The amount you have to eat when you have a fast metabolism like you and I, is CRAZY. You may think you eat a lot, but trust me, if you aren't gaining weight, you aren't eating enough and I used to think that I ate a lot too, but I know now I didn't. I know that sounds crazy, but it's true, but fortunately even with a fast metabolism it's still possible to gain weight (as I and many others have proved).
This may sound absolutely insane and will probably make a few girls in this thread feel really sick reading this but you have to eat in the order of 6-8 FULL, PROPER MEALS per day if you want to see any weight gain with a fast metabolism.
This means eating every 2-3 hours each day, literally. This combined with a decent weights program (no, you don't have to look like a gross freak from doing weights, because most people don't, but it's the best way to ensure you can gain weight, look bigger, look healthier and more attractive without piling on too much fat), will get results.
I realize this isn't a cheap option compared to the standard 3 meals a day either, but I can honestly say the money I used to spend on alcohol that now goes on extra food, is totally worth it.
If you (or anyone else, for that matter) want any more info on this, more detail on how I got such good progress, feel free to ask me here, or in that message thingo in my member profile.