Should I see a therapist?!

Absolutely! Going to therapy will help you discover why you bottle up your emotions, and what make you release them in an unhealthy way - your "triggers" so to speak. It is also not healthy to hurt yourself - although you probably do it to numb the mental "pain" you are going through. I recommend therapy to anyone and everyone, it helped me when I was 16, struggling with similar emotions. Check to see if your school has an onsite therapist, that is who I saw when I was your age. My parents didn't have to get involved that way. Good luck.

Self experience.

Talking about depression is one of the best things you can do to treat it. I'm a big believer that a problem shared is a problem halved. You definately have symptoms of depression and i think a psychiatrist would be a very good idea. Best of luck :)

When in doubt, and I mean serious doubt, talk to a therapist. It's always best to talk to someone, especially a specialist, rather than letting something go or left alone, and rather than aiming for "medications" instantly.

squeeze something sharp in hand and concentrate on the pain
get out and exercise, walk or run, gym, etc and work the feelings off that way. breathe deep and slow

its not always easy to be 16

yes you should. you may not want to but it would help you in the long run.

Seeing a therapist is a personal decision. If you feel you need to speak to someone about how you're feeling then its important you do so, regardless of what other people think. No one else thinks with your brain so how would they know? I would highly suggest giving it a shot, at least once. You have nothing to lose, you can only gain from the experience. If you're having suicidal thoughts then I suggest you talk to someone immediately. That's serious.