Please help me,i can't cope anymore.?

I'm 14 year old female, and i have been worrying for 18 months now,it's ruling my life,but it has never got as bad as this, my mum thinks i'm suffering from depression,shes also suffered from it. My doctors know, the hospital knows, and my whole family knows. Last night i even had to stay at the hospital because its getting VERY bad now, i can't deal with it anymore, i'm always crying about it and can't seem to just let it go out of my mind. I feel hopeless now,i feel i'm not worth anything anymore and i might as well just go now and leave this world because at least my mind will be at peace, i can't deal with it. It's on my mind ALL through the day night and when i wake up in the morning. I keep believing what my mind is saying,its like a voice where i'm arguing with it and it makes me so angry. Is the mind always right and is it true what its saying? i keep believing my worries are coming true and that what my mind is saying is true, what shall i do when it comes to my mind,how shall i calm myself down,and say none of its true. Please someone help,i can't live like this anymore.