How do I get back to "normal"?

Almost a year ago my dad was diagnosed with stage 4 head and neck cancer. I've been his caretaker for six months. (His only family is his sister, me and my brother) At first I was OK and just tried to help and do everything I could for him. (Dr appointments emergency visits take out the trash do laundry do dishes give daily pills take him where he wants to go etc) but for the past week all I've been able to think of is death. What it feels like to die. Knowing all of my loved ones will die too eventually. I as well will die. Knowing I won't be able to love the ones i cherish from the grave. This is all I think about. Nothing makes it go away. I've talked to a few amount of people about it in hopes that I'd get better... I lost the interest in eating a week ago. I can't sleep. When I do I only get 2 hours or 3 hours most then wake up and it starts again... I don't enjoy anything anymore. I'm 19 years old. Almost 20. And feel like im already halfway through my own life. I've watched my dad wither away. He has gotten a bit better. The treatment side effects are wearing off. He's so sad and angry all the time. Every time I see him I feel terrible.. which is everyday. I always knew that dying was a part of life. It never bothered me before but now that I've been around so many cancer patients I've had to witness it first hand. I can't even take care of myself. But no one else can take care of my father but me. My perspective on life has changed. I don't view people the same. Whenever I see anyone I ask myself why they're happy? I hate that I'm so cynical now. But these thoughts never go away. I spend most my time crying. I've always been overly sensitive. I don't know what to do but I cant go on living with these thoughts in my mind 24/7. Please help me?