I honestly.. can't bring myself to imagine how cutting yourself would help anything. I am aware of the medical results cutting gives (it in fact releases endorphins into your body for a short period of time; but the 'happy' or 'relieved' feeling dies out soon), but mentally, I wouldn't ever be able to do something like this.

People have various ways of coping with stress. Self-harm being one of them. The urge to cut yourself can be an addiction, even when the 'original' cause has already passed. I think PhantomLolita has a very strong suggestion with the rubber bands. I've talked with a few people with diagnosed depressions (including auto-mutilation) and the use of rubber bands is an actual advice we sometimes give. It might help you. (: (I tried them myself and holy hell it hurts)

But.. as you said, you have a lot of stress. Your friend isn't helping much by making things all about herself, and it seems you're quite upset about this as well. Perhaps you could involve your other best friend in a conversation about the feelings you're currently having as well as hers. We all have our problems, after all.

You're in the age group that's often subject to severe sadness (not depression) due to hormones. Stress on top of that can easily break someone down. Puberty is a confusing time, not only mentally but also physically (don't get me wrong, I'm not blaming it on the hormones here).

Perhaps, if you really want to know whether you're depressed or not, visit a doctor. Medication is an option that's on a free basis. There are plenty of other ways out there to help you (in your case, I think having someone to talk to might be quite a relief as well).

As of now, how would you like your life to be? What would need to change? And how could you make an active contribution in that change?