Ah, the wonderful teenage years when everything is the end of the world and rule of drama reigns supreme. Take comfort in the fact that the first relationship and breakup is also the hardest. Also try to remember that in a few months time (let alone years), this will all feel stale and rather trite.
Now, you said you've been seeing this girl since you were eleven. That's awfully young, my friend. Some people mature faster...but not that fast. And when you latch onto someone that young, it can be easy to grow dependent and lose your sense of self. I highly recommend you take some time for yourself. No one appreciates being single anymore, but really is a gift. Take time to appreciate yourself, your own company and your own abilities and interests. Not only will you be able to find happiness when you're in you're on your own, you're also be a healthier person for it, better able to be in a relationship when that time comes again.
Did you say you have a therapist? If so, he's the person you should be calling since he knows your history and personality. Us internet types can only offer our own experiences and none of us are health professionals (that I know of, could be wrong).
I'm not saying what you're feeling right now isn't real, it likely is. But if you're feelings are so intense it's not healthy. You're young, so a flair for the dramatic goes hand in hand with age, but if you're seriously contemplating suicide you should pick up the phone and call your therapist or your mother or a teacher you like. Any responsible adult you trust.