Dear Anna,
I received your email yesterday and i am so sorry for not being able to reply sooner. Now I have free time and I will tell you what I would do if I were you. First I would tell my parents about what I think and persuade them to let me study what I want. I would show them how good I am at fashion designing and may this would change my parents' opinion. If they still forced me to become a doctor, I would ask for help from my teacher or even my friends. They would tell my parents about my potential career and I think this matter would be solved soon.
Hope aforementioned solutions could help you. If you still have any trouble, don't hesitate to contact me.
Love,
Peter