Pamela,
I've been thinking about you all day. One thing popped into my mind while I was waiting for the work on my car to be completed. difficult child 1's therapist told us to always remain calm and unemotional when dealing with difficult child 1. He thrives on chaos. The more chaos he can create, the happier he is.
Since you said that you think your difficult child "enjoys" treating you badly, it reminded me of difficult child 1. Most of the time, I'm able to stop difficult child 1's horrible behavior by remaining calm and totally unemotional. (I wouldn't be able to do this without daily exercise and "ME" time).
For example, if difficult child 1 tells me to F-CK myself, I tell him in an unemotional voice that this sort of rudeness is unacceptable in our house. He is then given a check mark on his daily chart. This means that he has lost 15 minutes of his "Reward Time."
difficult child 1's therapist told us to keep our sentences to five or six words when having to talk to him. He said that difficult child 1 will totally tune out if we say too much. Also, if we try to explain to difficult child 1 that this sort of thing is hurtful, etc..., difficult child 1 will smile. In his twisted way of thinking, he thinks he has "won". If difficult child 1 thinks he has gotten to us, he'll continue to be as rude and defiant as he can be. He'll keep going if he thinks he can get us to yell at him. If we yell at him, he'll smile. To him, this is just a "game".
It's just an idea, but maybe it's worth a try in your house too. I have to admit that difficult child 1 got alot worse before he got better when we first started this approach. However, the results were worth it!!!
When he is being extremely horrible, I tell myself over and over again that nobody would want to be the way he is on purpose. I have to keep reminding myself that he is mentally ill. Sometimes this is harder than others.
Even now, difficult child 1 always behaves better for husband than for me. However, difficult child 1 knows that if he mistreats me, husband will not stand for it. When difficult child 1 is truly out of control, I have to call husband, have him leave work, and help me deal with difficult child 1. There are still times I can't do it alone.
Anyway, just a suggestion. Being unemotional with difficult child 1 has made my life lots easier. And, if you decide to try this approach, you always have cyber shoulders to lean on... WFEN