To add to what I said early, when I was young I had a horribly high metabolism, I tried all the weight gain promises stuff I could find and nothing, I ate more food then a pig, nothing happened. When I raced I had to stow away double the food bars that everyone else took because I got hungry too fast, my first race I packed the food that my team recommended which I ate all of it before the 50 mile mark of the race and had to ride into a mini mart at around the 75 mile mark because I was completely spent and had to buy two egg salad sandwich's, I was like that for a long time.
Just to give you an idea about my weight problem, I'm 6' tall, as a senior in High School I weighed 145 pounds! I now weigh 165 pounds, that's only a 20 pound increase in 44 years, and the last 5 years (when I turned 59 is when it started) I gained the most over a period of time and that was 10 pounds...but that might be due to going to a lot of places and eating out more during that time, plus during the winter months I don't ride as many miles, but age does and more than likely will slow down your metabolism. The weird thing is I put on those 10 pounds but in the last year it's stabilized and hasn't gone up.
You maybe suffering from the same problem I had, you know what I learned? DON'T WORRY ABOUT IT!! When you get older, I'm 64, your metabolism will slow down at some point, mine did at around 55 or 56, now I don't eat as much to maintain my energy, if I try to eat like I use to I gain weight, so I had to cut back on my eating which turned out to a be no big deal because don't need to anymore. Now I pack less food on rides and I'm good to go.
Be glad you can't gain weight because most people have the opposite problem, they gain too much as they get older, I have a friend who I've known since high school and he's gained 200 pounds since then! So if you don't have a thyroid problem like I mentioned earlier to be checked, be glad you have that "problem" as long as it's not making you sick which I doubt it is.