You remind me of myself when I was facing the same decision of what to study in poly. I had an interest in one of the arts courses, but realisticly, had nothing to show for it. I sucked in drawing and needless to say no portfolio of any kind. In the end I did IT because it was the closest thing I could develop an interest in.
Fact is, you don't know what you want to do next. Best advice I can give, you are still young, give yourself more time to decide on what you want to do as an adult. There are some people out there who were given a head start and they start being a highflyer early in life. Most of us aren't like that, we need time to find direction. Be careful what you reach for in case you fall flat on your face.
You still have to decide on which poly course to study, so sit down, research the courses and get a better understanding what each one gives. Shortlist those which sound promising, and work something out. It won't be easy, and it'll get very tough if you choose something you find no interest in. I say strive to finish the diploma, if you end up enjoying what you study, good. Else, at least you have a diploma of something. Good luck.