So if you are the mom, you will be best off accepting the reality that your man has moved on to a new marriage. Your kids now have a second female who will be a part of the caretaking team. It will only hurt you and your children to keep fighting this reality. You’re better off joining it.
If you are the stepmom you need to accept that your husband's former wife will be an on-going part of the family social system. As my own mother used to say, "A woman never marries just a man; she marries a family." Accepting that the on-going presence of the children's mother in your new family unit is a fact on the ground. You can negotiate how often and where and how, but her presence in some fashion is a given. Making the best of this reality will be to everyone's benefit, yours included.
What about the man's role?
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Hubby, listen up. Your role counts a lot. If you interact reasonably with both women, that can hugely help. If you are not listening to your new wife's concerns, or if you are unwilling to stand up appropriately to your former wife, you are being a big part of the problem.
To get to a better place, brainstorm together with your current wife, who is your teammate now, what each of you can do differently. What can each of you do to assure that both of you will both consistently sustain a calm businesslike manner in all your dealings with your children's mother? Note especially if you have had a history of submissive behavior to avoid her getting angry. This strategy may need to be replaced with more assertive standing up.
In any dispute, the role of third parties, in this case, the husband, can make all the difference. It's you the women are basically fighting over. So become an expert in win-win problem-solving. Then take the women’s tendencies to go to war as an opportunity to solidify your new marriage partnership and at the same time to soothe, or at least stand up to, your old.
If at least two of the three players in a step-family situation are doing their part, the odds zoom upward that everyone will calm down.
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Susan Heitler, Ph.D. is a Denver psychologist who specializes in couples counseling. Her book for couples, The Power of Two, is now available as a fun interactive online workshop at PowerOfTwoMarriage.com.