I need help getting over a plateau!?

so i started a losing weight the "healthy" way about 5 months ago and i was 140lbs. now i am at 123lbs and have tried every diet out there. my plateau used to be at 130 and i managed to get over it when i was going on a 1200 calorie diet. however, i do not go that low anymore as my metabolism slowed greatly. i have tried the gm diet, the skinny bit** diet, being anorexic, and am now on a zig zag diet where one day i will eat 1600 calories and the next two i will only eat 1200. it doesnt matter how much or how little i eat, i am always the same when the next week rolls around! i am currently lifting weights and playing volleyball twice a week and the other three days i walk about a mile. i am already quite muscular so i do not think i want anymore as i am trying to become a model. i eat healthy and whenever i have a treat i have a small bowl of frosted flakes because they are my favorite :) i only need to lose 5lbs and im in..but i really need help! i am completely against any eating disorders and still want to look healthy once i am finished. btw..i am 16 and 5'7