It hurts like hell now, but I don't think you ought to think about the situation in such very black and white terms. Intially, it seems helpful in terms of getting over someone and the pain of loss, but in the long-term it delays recovery as the feelings can turn into resentment, anger, hatred even or inwards to low self-worth, feeling unlovable etc.
For e.g. instead of saying, 'neither man wanted you' think logically and rationally.
At one point your ex-husband must have wanted you a great deal and loved you. He asked to marry you, and for a while at least you must have been very happy together. You shared a family together and brought two (?)DCs into this world. Now if that is only one good thing that came out of your marriage its a very very good thing. Your DCs wouldn't be who they are without your ex either, no matter how cold he appears.
Yes things didn't go as you expected, but sometimes the nature of a relationship changes. Whilst feeling devasted at losing your marriage, try and see it as a good thing you had, try and see how lucky you were to experience even the good times - things like these are what a life is made of. In time, after the acrimony dies down, you may well become good friends with your ex again. I know people who love their exs dearly - just differently.
The OM probably had strong feelings for you too. They just weren't stronger than the feelings he has for his wife. Had you met when he was unattached things might have been different. Although it is painful now, he at least has helped you realise what you do and don't want in a new relationship. Even had he left his wife, you would have had a relationship in which you would never feel secure and 100% able to trust. You would also have a double whammy of guilt, and step-children who would likely not appreciate you, all facts considered.
All this points to several things: you are to all intents and purposes, desirable, attractive, likeable and very lovable. You've made bad mistakes and decisions. Personally, I believe that the nicer a person is the more they feel the pain from any mistakes they make. Bad decisions roll off the back of nasty people.
So try and fill your life with a range of new experiences and eventually the pain will get less and less. It may take as long as 2 years or so, but one day you'll be able to look back without recrimination or hurt. I still say leaving your job is the crucial first step.