Yes, she is a cowgirl, if what you hear she has said is true. I would wonder then how much of you is competitive and refusing to dump him so she doesn't get him rather than realizing that you are finished with him? May be worth looking at at some point.
I think if I were in your situation I would think long and hard about my future and that of my babies. I would look at my finances, where I stand with child care and options there in, whether or not I could function by myself or where I could get help. I would consider what it might be like to find a man that is willing and excited about being a daddy figure in terms of being mono and staying in my life as such and then decide to move towards those goals.
Chances are that this "could" end. You "could" be alone and looking after your kids by yourself, at least part or most of the time.
Rationally and with some realism, all emotions aside, what do you need to do to make your life the way YOU need it to be. Him aside, just you. What can you do to raise your trust level in men/people, what can you do to become empowered, what can you do to raise your feelings of self worth and worth to others? Then act.
Even if this works out, you likely will be glad you did. You will be stronger in knowing that you have yourself to rely on, no one else.... them be damned. You have your own power within you to stand on your own two feet and show yourself, the world, and most of all your kids what it means to be a confident, proud and amazing woman.
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