Q: I’m having a problem with my child’s mother and I’m not sure how to handle it. She continually undermines my plans, but this last time took the cake.

I’m a huge baseball fan and have wanted to take my son to a baseball game all his life. His mother knows this. He’s been too young, but this year, for his fifth birthday, I got tickets for a game a week after his birthday. I was very excited about it and told his mother — I even discussed the possibility of her joining us. She then secretly purchased tickets for the day before the game and presented them to our son a week before his birthday.

How do you co-parent with someone like that?

A: There’s more going on than simply an issue with co-parenting. What you describe took planning and preparation to undermine your plans, and aside from bad ex-etiquette, it’s an indicator that Mom may be locked in a secret battle where she sees your child’s love and devotion as the prize.

Manipulation to this degree certainly does not put your child’s best interest first, and I always hope when I hear this kind of thing that it’s done out of ignorance and not by design because if it’s by design, there may be a mental health component — and that makes things twice as difficult.

So, how can you compete when a co-parent acts so underhandedly?

Start by setting appropriate boundaries, which can be both physical and emotional.

Physical boundaries are making things clear about the other parent’s interaction when your child is with you. This may include things like adhering to the schedule, regulating the times for phone calls, or stating that all changes in schedule shall be documented by e-mail.

Emotional boundaries are about you and what you will tolerate in your relationship with the other parent. It’s about getting clear in your head how you want to act and curbing feelings that put you in a reactive rather than a proactive state of mind. Make sure you make your decisions in the best interest of your child — including how you interact with his mother. Learn to walk away from the conflict, especially if your child is watching.

On a practical level that means you see what your child’s mother did, tell her you will not engage in the tit-for-tat behavior, and take your child to the baseball game. Make your own memories. If your child talks about what a great time he had with Mom, see that as wonderful, not as competition.

Hopefully, you will get to the point where no explanation to her is necessary and you automatically don’t engage her bad behavior. It simply becomes more important to be a positive role model for your child than win the battle with your ex.

Jann Blackstone is the author of “Ex-etiquette for Parents: Good Behavior After Divorce or Separation” and founder of bonusfamilies.com.