Shari, sounds like your stress level has hit maximum overload. The difficult child factor affecting your job is difficult at best; devastating in many cases.
Is there a way you can take a long weekend away - ALONE? You need a break.
If not, go to the doctor & ask for something for your depression/anxiety.
How about respite resources? Can husband handle the kids while you head across town for a night in a motel?
You need to rebuild you - your difficult children are a marathon, not a sprint. Pace yourself.
For me, I had completely shut down. I could no longer keep track of things, was losing sleep, couldn't remember names or my way home from work. I could tell that family stressors were taking a huge toll on me.
I'm on a new antidepressant & a new anti anxiety medications along with weekly therapy. It's helped. It's also helped that I'm currently unemployed & can pace myself to keep up with the demands of the tweedles & still have time for myself.
Just throwing some ideas out your way.