Feeling down.?

Okay so firstly I am a 17 year old female who is currently in college studyingwhat I believe is my desired career. Over the last number of years I have been feeling down. Not a wave of sadness at a particular time but a constant, nagging feeling of not being fully happy. It's like I could be in the middle of watching the funniest film but have a feeling of unhappiness in the back of my mind. It's like it's always there, not always overbearing but always just there. I am always trying to remind myself how lucky I am, my parents are amazing, I have a lovely home, I have everything I could wish for and more but I still can't shake this feeling of sadness. I don't even know if it's sadness, the best way to describe it is like a cloud, following me around making me aware that I am not fully content. I really don't know what to do anymore.