writer progress

I have never found myself to be what you call a "good writer". in my mind i think i am writing a really good essay, or summary. but when i get an evaluation back from my teacher it's just red marks and red lines and corrections everywhere! BUT, i feel like you can be a good writer, if it's something interesting to you, or if it's something about your life. i feel as if i am better at writing an essay if i am given the topic and the guidelines for the material. if i am just given a person, and how many pages it has to be... i will be stuck on it forever and ever. i look back at my previous writings from highschool and i will admit i have gotten a little better, maybe it just takes time.. and for some of us it takes a little longer than others!