Would you ever in a million years marry the father or your children again, OP, knowing how impossible it is to predict him? There is no way. If you got yourself to a different state than he lives in, the chances are that you would stay there, giving thanks for a bullet dodged. You were married to him for 15 years, but if anything this woman has been given even worse treatment. What honor has he given her? Essentially nothing, and she has hung around a dozen years and yet keeps welcoming him. If she gives as bad as she gets, that only makes the situation even LESS one that any sane person would venture into on purpose. A dozen years, and it makes the news when he decides he is going to “date her exclusively” instead of messing around with all sorts of other women. Oh, wow, stop the presses, that is almost like a COMMITMENT!
The influence of the childrens’ father is far more of a concern than the influence of this woman, IMHO. He is not your husband, and your attempt at marriage was not valid. You can only hope that any woman for whom he does not do better will be spared the same treatment you got. It is the kind of treatment none of us would wish on an enemy, at least not except when we have days that call for repentance.
IMHO, a good start would be to stop calling her “his mistress”
Well, and he may have his current girlfriend plus a secret “mistress,” too, unfortunately. This fellow carried on multiple concurrent adulteries? That is a hard habit to break. Plus a restraining order? You have to wonder about this woman, dating a guy who messed around on his wife to date her and against whom she herself has had a restraining order in the past. Fast forward 12 years–she’s dating him again? That it is deemed “exclusive” on his part makes the evening news? Does this show great judgment?
Does she not remind you of the woman at the well, the woman who had been married five times, living with a man not her husband, in a society where women were not allowed to initiate divorce? Either she was widowed five times (not likely, based on the context), or else she was rejected as a wife five times, then lived with a fellow who would not marry her at all. It is not surprising that she was at the well at noon, and alone, rather than earlier in the day when the other women went to fetch their water.
The chances are very high that she needs the living waters of Christ. If her relationship with your husband goes down in flames (which is unfortunately the way to bet), then you of all people will know what she has been through. Don’t let your resentment of her keep you from feeling any sympathy for her, OP. Your ex-husband was not lured away by all the seductive women coming at him from every side. He went out looking to please himself, not concerned that he was betraying your trust. Pray that she gets far from him, for her own good. Pray that she be given the light of Christ, that she respond with joy when she is confronted in the depths of her heart as the woman at the well was, who said, “Come see a man who told me everything I have done.” (John 4:29)
How to cope with her? Start by being glad that you will never have to change places with her. You have been delivered from a faithless man, and you have been preserved from a life of infidelity. You do not have any self-generated grace to account for this, but the gift of the grace of God. The measure you measure with will be measured out to you. Extend mercy to her, and mercy will be given to you. Every one of us need it. You have a chance to show that you have the gratitude to warrant it. Hang in there; even the strength to have gratitude comes from God. Ask for the strength from the God who has saved you and brought you to this day. Let him meet her “at the well” for you. Try that, and see how it goes.
I had a relative of whom it was said that nothing worse could be said of anyone than that this woman could think of nothing good to say about them. If she would say nothing, there was nothing good to be said. You don’t have to excuse what this woman did to you. You can say, “Twelve years later, and she is not happily married. Twelve years later, and it is news that he is going to devote himself to her alone. I wish her well, because she has gone through punishment enough. Don’t ever let yourself be treated like that and don’t ever ever EVER let me find out you ever did anything like that. People you love deserve better.”
What about mercy for your husband? Pray that he have a true conversion. He needs to renounce the glamour of evil and accept the Gospel. Date you again, though? Not if he were the last man on earth. Never. No way. There is no reason whatsoever to grant him that position of trust ever again.