In a habit of binging (and sometimes purging) Advice in losing weight in a healthy way?

Hi,

I'm 17 years old and and  pretty obsessed with food and weight. I find i can't stop thinking about it all the time. I am 174cm and my weight varies between 128pounds and 138 pounds. My target weight is about 117pounds and i know if i reach this i can stop obsessing about my figure and appearance and weight and actually live my life properly. After being about 143pounds i managed to go down to about 132pounds after becoming more conscious and aware of what i ate. I was then really happy when i got to 128 pounds. However, recently I've been putting on weight as i go through habits of binging, like literally eating packets of cookies and stuffing my face with food. Sometimes i throw some of it up. Whenever i start doing well with my eating (being healthy/ eating soup etc) and lose a bit of weight in like 3 days i suddenly can't help but eat loads more in the next few days and i put on anything I've lost. at the moment I'm about 132pounds but i know its going up and i can't help but waste hours of my life in the bathroom forcing myself to be sick. I know its stupid but i just want to lose weight, back to the 128pounds and eventually to 117pounds. My family aren't helping me to lose weight cos they say i don't "need to". yes I'm not overweight btu theres nothing wrong with wanting to feel happier about myself.

(also being within a group of friends who are pretty messed up about food is difficult too. My best friend is battling anorexia, my sister doesn't eat anything and and one of my other closest friends is bulimic. we all try and help eachotehr and encourage each other to eat, and so i end up having to eat more to encourage my friends to eat more cos i am worried about them. So i find it hard at school to diet and eat salad not only cos it sets a bad example if youde like to them but also cos once i start eating i just can't stop eating more and more.)

I really want to be happy with myself and have a nice figure i'm proud of and wear shorts and bikinis by losing 10pounds but i don't know how? I spend all my time thinking about this and i don't know what to do. Does anyone have any advice for me? I don't want to throw up my food i know its horrible but i can't stop it because  i don't want to gain weight, which i am. :( How do you motivate yourself to eat healthily and not "yo-yo" ie. i go through days of binging and days of starving myself.

Sorry for the rant i just had to let it out cos i am feeling so depressed :(