“I have 15 years. I live in a foster family and this is my problem. My mom keeps talking, that if she had a daughter of her own, she would be completely different from me, would surely be better, more intelligent, prettier… I can't cope with all this anymore.”
I'll tell you something, which will sound very brutal. Well, children most often suffer from the stupidity of adults. Your surrogate mom is clearly a stupid person. I hope, that realizing it will allow you to survive with her right up to the point, when you can walk away from her.