But that's what I was trying to get away from...

When divorcing a spouse without any children in the picture, you are probably never going to see that person again. Not unless you go out of your way to do so. But when you divorce your spouse and you share children, the story is much different. You will spend the next X number of years seeing that person, probably multiple times a week, or emailing/texting them even more. There may even be the occasion that requires a phone call.

If your divorce was amicable, you might not understand why this may be a cause for stress and tension.  If your current spouse is supportive and wonderful, they will listen to what you have to say, or try, and get you to tell them what is bothering you. I'm lucky that way, my spouse is very supportive and wonderful. She helps me through the tough patches with my ex.  It's not that I can't deal with them, but rather that I try to internalize them so they don't cause strife in our lives.

My divorce was less than amicable, so there is a lot of tension, even 3+ years later.  Only time will tell if the tension will fade.  I'm really hoping it will but I can only do so much.  You quickly learn that you cannot make someone work with you. You cannot make someone co-parent with you. No matter how much you try, if they want to do things the way they see fit, you will not affect change.  Instead, they will dig in their heels and be even more difficult. 

At the best of times, you will not let this affect the children. The kids don't need to know how difficult their other parent is being, and they probably don't even know what you are talking about anyhow. Then there are those other times.  You are not at the top of your game, you've had a rough day at work, or you've just told the kids something for the seemingly millionth time, and you lose it. The following action of theirs was exactly something your ex used to do to you, and it grates on your very last nerve. It could be something as simple as the child's body language, but it is clearly a mannerism that your ex had/has. What do you do?  Internally, your blood pressure spikes and you start screaming in your head that you can't believe they have learned that behavior from HER!  Outwardly, you try not to let your feelings show and tell them that is not a behavior we have in our house. They will have no idea how their action came across and how it affected you. And you don't let them know. Bagging on your ex will cause resentment and more behavior problems.  

The sooner you realize that they are going to pick up things from your ex that drive you insane the better.  Conversely, they will pick up things from you that will drive your ex up the wall as well.