Don't work with PD's then.... at least not until you get a thicker skin about treatment failures. While certainly you should examine your treatment decisions and approaches in a healthy honest way, you are not responsible for the treatment success or failure of a client.
One supervisor put it this way for me: "You don't control the outcome, you are simply not that powerful." As soon as you start believing that you control a client's behavior you have crossed a line that is no longer therapeutic and is manipulative, or at the very least arrogant and unfounded.
The sooner you admit to yourself that you will never "cure" anyone, the sooner you'll feel better about the outcomes. You'll find that in many cases you can make a difference, when you cannot, refer to someone you believe can make the difference. That is the ethical thing to do, and it sounds like that is exactly what you did do. Do not place a value on treatment outcome, but do place a value on delivering treatment competently, you can do everything right and still end out with a horrible treatment outcome.
Do not make treatment outcomes about you... it will drive you nuts. Now tell us, how powerful are you? Is your Kung Fu better than mine?
Mark