Believe it or not, your size has little to do with your metabolism. Actually, your metabolism is as fast or as slow as you make it. The more physical activity you do, the faster your metabolism would be. The less physical activity you do, the slower your metabolism would be.
In other words: the more you do, the more you burn. The less you do, the less you burn.
However, just because your metabolism is fast, it doesn't mean you're going to be skinny. It doesn't even mean that you're going to lose a lot of weight. You could have a fast metabolism and still be obese.
The amount of fat you have has more to do with genetics than it does with the speed of your metabolism. Some people naturally have little body fat. These are what we call "ectomorphs," the folks who are naturally skinny. If they lift a little weight, they would look muscular because they have little fat to hide their muscles.
Then you have those who are naturally prone to storing a high amount of fat. These are your endomorphs. These folks are curvy. They could easily get fat if they are not careful.
And for the folks who are in between, they are your mesomorphs (meso meaning middle or in between). They have about a "normal" amount of body fat. They have more muscle mass than your ectomorphs and sometimes more muscle mass than endomorphs. The mesomorphs could lift weights and look very muscular because they have a lot of muscle mass.
With all of these "morph" groups, none of them have a fast or slow metabolism. It's just that their bodies utilize fat differently. With ectomorphs, they already have a lack of body fat, so it would be hard for them to get fat from eating a lot.
Endomorphs have more body fat than ectomorphs, so it's very easy for them to get fat if they eat too much.
Mesomorphs could actually get fat easy because of all the muscle mass they have. More muscle needs more support from fat. This is why bodybuilders could gain a ton of fat after their bodybuilding competitions. All that muscle needs fat for energy.
Now question: doesn't your metabolism slow down as you get older?
Yes it does -- but only because you don't do anything! Heck, your metabolism could slow down after you stop growing! While you're a teenager, your body is blasting out growth hormones, which is why teenagers seem to drop weight so quickly, especially when they are exercising. But once the growth hormone production stops -- typically around 16 to 22 years of age -- then your metabolism would also slow down. If your body is no longer growing, then there is no need for a fast metabolism.
But if you're not doing any regular physical activity, then your body's metabolism will slow down after you stop growing. After you hit 22, then say hello to your beer belly, thunder thighs, and double chins!
Remember: the more you do, the more you burn. The less you do, the less you burn.
So, if you're overweight but you're exercising like crazy, then it's not because your metabolism is screwed up. From experience, the people who have started losing serious weight are those who increased the intensity and the frequency of their exercises -- in addition to changing their diets.
And if you feel that you've done all that you could to lose the weight, but still seeing barely any results, then you may benefit from having a professional trainer. Yes, I know, I know, you don't want to spend the money for help. But from experience with working with clients one-on-one, people have always gotten better results working with a trainer than trying to do things on their own.
Don't blame your metabolism. It's working just fine -- whether or not your overweight.