Can't lose weight through diet or exercise?

I'm 27 years old, I've been overweight since I was 10. 5'4, 275 lbs. I'll give the quick run down of the usual questions I am asked.

No my family isn't overweight, I am the ONLY person in my family who is overweight. The rest of my family is rail thin.

I've had a personal trainer twice in my life who have put me on different diets.

I've done; Atkins, The body Diet, Weight Watchers, Eating 5 times a day, 1200 calories a day, strictly cardio and tried a vegetarian diet.

I've had my thyroid, hormones, vitamins, and cortisol levels checked, nothing seems out of place with them.

I've been told multiple times that I have great blood pressure (not just for an obese person), haven't had issues with being pre-diabetic, and even was told I'm more healthy than most think people who go to the doctor. (told this by the doctor).

What am I missing? I've heard of a hormone test done by spit? I'm just desperate for anything. The only time I've been able to lose weight is by not eating at all, and that's miserable.

Please help :/