My daughter "Brooke" is a senior in high school. Her mother and I have been separated six years and divorced for three. We have a cordial relationship and I am allowed great visitation with my daughter.
I have had a girlfriend for 18 months, "Michelle." We plan on moving in together in April, and I fully expect to invite her to Brooke's graduation ceremony and a party I am hosting and paying for. My ex-wife adamantly states I cannot bring my girlfriend. I think she may use her position as the parent who lives with my daughter to influence Brooke to agree. I don't think Brooke cares. She has met my girlfriend many times and even spent the night at her house once.
How do I handle this situation?
Answer: Divorced couples who remain connected as co-parents eventually have to accept each other's new partners, for the kids' sake if nothing else: They need to see you both let go of grudges, even the legitimate ones.
That said, having a valid point doesn't give you license to dig in - especially not months before the party and before you've run it by Brooke. Your cordial divorce suggests you know this, but I'll say it anyway: It's better to be decent than right.
Because this is your daughter's celebration, not yours, and because your ex-wife's adamancy suggests unhealed wounds, the decent move is to try peacemaking first.
Have you asked why your ex is so insistent? Nonconfrontationally, by acknowledging her feelings as valid?: "I know it's difficult. I'd like to think I'd be welcoming of someone new in your life. May I ask why the strong objection?"
Then listen. Give empathy a chance. Then, assuming you don't get the answer you want from your ex, you drop it: "I'll let this rest, and try seeing it from your perspective. I hope you'll do the same for me." Then use these months to be the model of cooperation vs. I'm-right exasperation.
If your ex remains adamant next spring, then offer compromises that don't hit bone. Michelle skips the graduation, for example, but attends the party with you. Or don't bring Michelle, but be clear it's the only time you'll exclude. Why? Because a kid's rite of passage is a lousy time for parents to road-test their contentious, new-partner reality for the first time. You'll still be right next time.