How to extend my vocabulary? 15 year old female?

i'm 15 years old and i think my vocabulary really needs to be extended. I write, or at least, I Love writing. I don't know any big words, i've never been to school, but my parents have put us onto educational games and taught us other forms of education. I am the only one from my 5 other siblings that are actually really good, and since I've been home schooled like them, i should be knowing what they know too. But i just don't have it. If i have a deep feeling or something i won't be able to describe myself. Whatever I write, it has basic english. Many people compliment my writing, saying that it has great vocabulary, it is simple and understanding. But they do not know that, that is as far as i can go. The probably think that I have a vast range of vocabulary and I write that way purposely just to get everyones understanding. I feel slightly embarred when people expect me to write. They think i write really good. But soon they will notice that i use the same words often. I don't know more than that. What should I do? I really don't like being so backwards for my age. I'm 15, with this amount of vocabulary, it's .. humiliating for me! Please suggest to me something that will help me! Thank you so much!