Do I need to see a doctor?

This entire last spring and start of the summer I have been unhappy. My boyfriend is changing colleges, my best friend has her own life, my brother is 2,000 miles away from me, and my parents are oblivious to the entire world. I haven't been happy for a long time. I shouldn't say that I have been unhappy, but I don't really feel. I feel sick all the time. My chest and my back feel heavy, like I can't breathe. I tried writing down all my feelings in a book, and then my boyfriend read it and is mad at me. I wrote things I probably shouldn't have, but that book was the only way I could vent. No one else was listening. So now I can't write in my book anymore and I'm not sure what to do. I used to workout all the time and eat really healthy, but I can't even seem to do that anymore. I've gained weight and my acne has come back horribly worse than when I was younger. I'm 20 by the way, turning 21 in October. I just have no motivation. I feel like I have nothing to look forward to in life. I go to college and work during the summer and school year, but I just don't feel like I have a legitimate purpose. Does anyone have any ideas on how I can move out this horrible phase of my life?