I weigh 170. I want to drop to 150

I am a 16 year old girl. I weigh 170. I want to drop to 150. I am having trouble because I lost so muvh weight and I am now plateauing. I have my boyfriends prom in three months an I want to be the girl that I always imagined him with. Don't get me wrong he thinks i am gorgeous, but I want to be able to see myself in the way he sees me. He is going off to college soon and it is really important to me that I look healthy and beautiful before he goes off. Please Help!!!!!