What's the point in her parents being alive and not dying of cancer if they're bitches to her? Seriously. wtf. That's not something that you can use as an argument to prove that her life is not too bad. She isn't getting any benefits from them not having cancer currently.
And I don't see how it matters to her that there are people who have it worse. Other people having problems does not make her problems any smaller or less hurtful.
Having food and a place to live does not mean you should, or will be happy. It's not enough. If it was, no one would even bother to form relationships, friendships, or basically to do anything but eat and sit around. But food and roofs don't make people truly happy nor do they fulfil someone's life.
It's stupid as fuck to even think that it works that way.
Most people need much more than food and a roof to be happy.
And if she doesn't have what she needs for happiness, it's perfectly normal to feel bad about it.
You need new friends, Civilian. Your parents are not really kind or supportive, I'd say, considering the fact they want to kick you out as soon as you're legal. Friends that you have can't even be called friends. Try meeting new people and befriending them, because that's what you need now - to change the group of people you spend your time with. When you do that, you will feel differently because the people you're with affect you a lot. You need support, and people that care about you. You're not getting that from your parents or your so called "friends", so you should get new people in your life that will not make you feel bad as the ones you have now do.