I' m an 18 year old girl. I always feel like I need sex, like I have to have it and I can't not have sex. Why do I feel like I need it?

It's your hormones talking. This is when you need to learn how to sublimate -- to find useful and productive activities to get involved in, so that those urges don't control your life.

Payday candy bars are my passion, my constant longing. I always feel like I need Payday candy bars in my life, like I have to have one and I can't not have one. So, what would happen if I ate one every time I lusted after one?

You're at that age when you feel that way. My advise is to find something else to do to fill your time because sex leads to pregnancy and that a whole other ordeal. But if you can't shake the urge, MASTURBATE!! Self love is the safest sex if you need it that bad.