Hi Jess, you are not alone. However your really need to go and speak to the doctor. Believe me your Dr will have heard this all before you are not the first. We all understand this here, personally I feel very ashamed of my situation, and only started to feel better about myself by speaking to the Dr and those closest to me. Looking back to my teenage years I can now admit that I was depressed then, for certain family reasons, and had all the unreasonable thoughts similar to you as I am sure a lot of others will have on this forum.
You have also a good basis to work from, you said you have a stable family life............believe me now as a parent there is nothing like the love of parents for there children. Actually the love of my husband and the imprint of my 2 sons stay upmost in my mind. Try to keep the positive nice times in the forfront of your mind when things are really bleak.
It has taken a bit of time but now personally I am feeling a little better each day. People never really see other difficulties unless they are visible, but I know the best thing was that I never met my best friend until I was 18, I select my friends carefully, and now have support from them. Both my sons by the way say they only met their true friends after leaving high school.
Keep the nice images in front of your head when in deep despair and go and see your GP. %^&