You have to get through it - as someone who suffered from anorexia and bulimia I can say that getting obsessed doesn't help anything. See a councillor if you're really worried - you just have to find some confidence somewhere... I mean I'm not completely secure, but I'm not as horrifically obsessed with the way I look as I used to be. Weight doesn't determine how pretty you are anyway, when I had anorexia my face was horrible and sickly looking, my skin was so pale and had patches on it from calcium deficiency. Not attractive.
Now I exercise instead of doing stupid fad diets, I feel a lot better for it - may not feel 'thin' but I feel healthy and I think that's far better. Also I cut out most artificial sugars and fats out of my diet, I feel a lot better because of that too - mind you, I think that's still kind of obsessive behaviour but that's what helps me feel comfortable at the weight I am now.
I think most girls like to look like they take care of themselves by wearing makeup and do their hair, I mean as long as you aren't caking the makeup on I don't see the problem. However, if you're not doing it for yourself (like I do) don't feel like you need to wear makeup. Don't pretend to be someone you're not just because you think other people will be impressed by it otherwise you're living a very fake existance.