SP - In regards to your stbx and her behavior...I recall at the early stages of her WAW-dom that you asked a question along the lines of "but what will we tell the kids..." and her answer was something like "tell them because mommy is a f-ing bitch".
Seems she wasn't exaggerating, was she? She really meant that, and seemed to be being honest, not just flippant.
Would it help you at all to allow those words of hers to ring in your ears as you come across these low times in your parenting?
Remind yourself of her words everytime you've got a crying child who WAW has disappointed once again.
Her honesty (by saying the above) may be the key to your being able to get through this stuff in tact.
Once the kids are older and can relate to her on a different level, they will know what the truth is anyway...you won't even ever have to tell them.