How can I force my body to be skinny when eating right and getting plenty of exercise leaves me fat?

I was fat in high school, even though family meals were well balanced, few sweets, and I walked everywhere, rollerskated, played tennis, rode my bike. I was a size 24 in college in spite of walking and hiking plus not being able to afford to eat anywhere but the caffeteria. I was a 24 after college, working two jobs - waitressing, cleaning - and walking to work, and being a vegetarian. I'd gain weight when not that active, got down to a size 24 following Susan Powter's book, size 24 on The Zone, size 24 on Atkins, size 24 on Overeaters Anonymous while living in the desert with no car riding my bike or walking everywhere plus taking tai chi cles. I've gained more weight, but I need to know how to force my body to actually be skinny before I lift another finger - I want to be a size 12, if I can't be skinny it doesn't matter if I'm a size 24 or a size 572. I'm already on synthroid, and no, my doctor won't increase the dosage, yes, they've tested to be sure it was the right dose.