How can I become better at composition writing?

I got an F in almost every English class in high school, and because I wanted to get better at writing, I decided to major in the subject when I got to college. I’m almost done with my B.A. but I’m still not very good at writing essays. I understand the basic structure of a 5 paragraph essay, but after that I’m lost, and I just don’t know why the process still isn’t coming easily to me.