How come at 17 my metabolism is so slow?

I am a 17 y/o girl, and am 5'4 and 140 lbs! last year, i weighed 127 lbs (but that was VERY hard to maintain and would constantly creep up to 132). Now i am pushing a f***cking size 8 (mind you i am a teenager)! I actually eat less than last year (just a little less b/c i am less hungry), but was put on some stupid lithium mood stablilizer medication. i went to a special doctor b/c my periods were irregular, and she said there is NO way to lose weight on this medication, unless you do INTENSE AEROBIC exercise. for 3 years, i have been complaining to my folks about my weight, and they say that i need to EXERCISE more. (their definition of exercising is sweating buckets and ping out). a few times a week, i go for 30 minute walks (where i live is very hilly). i used to lift weights, but now i feel there is no incentive. But the thing is, even b4 these meds, i would litterally have to STARVE myself to lose weight. Now i am afraid to starve myself b/c that slows down your metabolism. the irony of this is that i eat soo much healthier than these bimbo toothpicks at school. they eat bolognea sandwiches and 3 greasy chocolate chip cookies, meanwhile i am eating a salad w/ chickpeas and fat free macrobiotic salad dressing and an apple for a snack. i do not eat refined carbs or cheese for the most part. i NEVER drink soda/pop or even juice for pete's sake! ONLY water or pure green or chamomile tea. i always eat breakfast unlike these skinny b****tchs at school. i am pissed, b/c my $200 pinstrip juicy couture jeans that i love do not even fit over my thighs anymore. i ask my parents to f**cking stop buying/ baking cakes and pies, but my daddy said that it is a "weakness of character" to loose control when eating these things. he said i just need to develop dicipline (when we have these things in the house, i usally have one realatively large piece everyday until it is gone). i do crave sweets, but i only have one piece, while the skinny girls at school eat 3 f***cking cookies at lunch, chips for a snack along w/ their damn bolognea sandwiches on sh***ty WHITE BREAD! My weight makes me feel terrible about myself, and i do nothing but obsess over it! i feel like an unglamourous girl, and what will happen when i am 50!?! i feel it is totally out of my control even though instead of doing my homework i spend two hours researching weight loss tips. i want my new doctor to perscribe me aderall, and i will coax him to do that (and get me the hell off of this lithium sh***t.) Why is my metabolism so damn slow at 17, and do most 17 year old girls have this much difficulty losing 15 lbs?