When is metabolism a medical issue, not a lifestyle choice?

I have been overweight my entire life, literally for as long as I can remember, by at least 75-100 pounds my entire adult life. No amount of moderate exercise, water, or calorie counting has ever made more than a 20 pound difference. By the way I qualify moderate exercise as 30-60 minutes 5 times a week.

Combine that with the fact that my bowel movements have never been more than once a week, and sometimes not even that. I have tried various fiber products, increased water intake, and rufage consumption, none of it changed anything. Small to medium amounts of food go in and barely anything comes out.

I don't over eat. I have seen shows on TV like the biggest looser where it shows the people ordering 5 burgers and a diet coke, that's not me.

I just turned 37 and I'm gaining weight, what little metabolism I have seems to be disappearing and I am already 300 pounds. I have spoken to my dr and he told me to basically drink even more water, more fiber pills, laxative and checked my blood work for thyroid issues that weren't there.

I know the American population is really into easy fixes with a pill, but isn't this medical not lifestyle? Or am I missing something?