If you feel that you are depressed and it's really starting to negatively impact your well-being then I would try to find someone you can trust and talk to them about it. Parents, teachers, therapists, friends whoever as zel said that can help you out and get you back on track. Late teens can be rough, I'm a year older than you and I've had my fair share of rough patches in the last couple of years. It's understandable I think, people our age are trying to find out who we are, dealing with a lot of new responsibilities as a result of starting to enter the real world and on top of that university/college can be rather stressful. There's a ton of stuff going on and it's easy to get overwhelmed. Overall it's a learning process, figuring out how to deal with stresses better, looking towards the bright side of life and taking small steps to make some positive change.
I do wish you the best of luck and hope that things can improve for you.
