Feeling miserable, trapped and confused

I'm so sick of feeling like this.

I am feeling like I would do almost anything to get away from my stepdaughter. I want to leave but at the same time it makes me feel sick thinking about it.

How could I leave my husband? How could I take my children away from their daddy, and not burst into tears when Jimmy cries for him and asks when he's coming home?

How can I live on my own? How can I be away from my children on weekends? How can I survive in an empty, lonely house every weekend?

But how can I go on like this? How can I keep living in misery, being mother to a child that I can't stand, whose own parents find it very difficult to like her? A child who, at 12 years of age, whinges constantly, is lazy, selfish, stubborn, and nasty? Who gets worse and worse every year, with the worst years yet to come, and whom 18 pyschology sessions seem to have been completely wasted on?

How can I keep doing this?