Funny (as in strange coincidence) you mentioned that difficult child's are God's messengers - who are hear to teach us too.
When N* came to live with me, people would say, "She's lucky to have you." I would always reply, "I'm lucky to have her." And I meant it. I started on this journey thinking I had plenty to teach her. However, what I've learned has far surpassed what I've taught her. I thought I was patient, tolerant, loving, forgiving, caring, compassionate, strong, generous, etc. before N* came to stay permanently. Wow! Did I ever have that wrong!
I've grown, in ways I know I would not have, if N* had not come to live with me. I know I'm capable of unconditional love and a warrior spirit - simultaneously.
On a lighthearted note, while I would not take a million dollars for N*, I would not pay anyone a nickel for another one just like her. I had hoped for better things for her, for her sake, not mine. I've had to learn to accept the things I cannot change. That's been a tough lesson for me.