How can I improve my writing?

I am a freshman student wanting to better my writing skills. I used to somewhat enjoy writing but now its a task for me and it is a lot more difficult as well. I want to start writing again, but now whenever I get around to it, I really don’t like how it turns out; whether it be book reports, essays(especially these),prose, and even poetry.
People I know and even my teachers say I’m an excellent writer, but I feel I’m not living up to my potential and I want to be the best I can be right now. And I really need to perfect my writing soon, as I am entering a major writing competition. Anyone have suggestions, tips, guidance, experience, information, or websites that can help me?