At What Point Do I Give Up?

When I took in my foster son three and a half years ago I thought I was going to become his gaurdian or adopt him. He has no parents, as they were killed when he was ten. Any ways, we have had our ups and downs, too much to write about, but long story short. He is now in high school and has been ditching all of his classes. And the rule has always been that school is a must. He speaks very disrespectfully to me (profanity and name calls), he has punched a hole in the wall of his bedroom (and I rent! there goes my $2000 deposit). It has gotten so bad that I am depressed often and dread coming home. He has also threatened to kill me for telling his social worker what has been going on. His therapist seems to think he doesn't mean it, but still, he shouldn't be making these threats, right? I'm a single foster mom who is burnt out. Things had been good for some time, but now it is a down hill roller coaster. I read every where not to give up on these kids because then you are rejecting them just like their parents and other homes did. But what about my sanity and mental health? When do I say enough is enough. Besides that, he demands things. For example he got mad the other day and threw his cell phone across the street. Then he has the nerve to ask for a new phone- from me! I tell him he needs to get a job and buy his own. He also lost his IPOD (last years bday gift) and demands I also buy him a new one and then he will go to school. f course the answer is the same as the last, get a job and buy your own. He takes no responsibilty for his actions. I am so frustrated! Any advice??? I'd appreciate it.