Divorce hurts. It is your natural inclination to move away from the hurt that divorce brings as rapidly as possible. If you never had to see your ex again, that would make the hurt dissipate. But your children deserve to have and enjoy the only parents they will ever have, regardless of what your feelings about your ex might be. So you can't move away from that particularly distasteful aspect of divorce: interfacing on some kind of regular basis with your ex because the two of you share custody.
Because you care so deeply about your children, about their success, and about their needs - in fact, you place them higher on your list than yourself most of the time - you are simply forced to find a way to make the interface with their other parent (your dreaded ex) work so that they don't feel any of the disturbance between the two of you.
How do you create a harmonious relationship with someone you are divorcing for the sake of your children? Here are some suggestions:
Keep Track of Their Good Points. You know them. You once fell in love with him/her because of them. They haven't gone away despite the fact that he wants to divorce you now. He still has those good points. You can choose to focus on those good points and allow your children to discover other aspects of his personality as they naturally mature. Kids deserve to love their mommy or daddy without any negative feedback.
Remain Objective. If you permit yourself to fall into entertaining only those ideas you whirl around inside your head, you've succumbed to "subjectivity." Of course your opinion is nasty! You've been hurt. Your kids have not experienced this nastiness from their other parent though. And they deserve to not know your nasty thoughts. They deserve to see their parents as simply powerful and wonderful. So set your nasty thoughts aside for their sake and remain objective about their mommy or daddy.
Don't Make Excuses - Be Factual. Okay, the ex has promised to pick up the kids for a weekend outing. You've got them bathed, slicked and ready. They're waiting on the couch. They've been waiting for an hour. He's past due and they're getting rambunctious. You call him. He "forgot" about picking them up. You know they are going to be disappointed. You don't have to say what a louse he is. You put them in the car, take them to the park yourself, and say "Daddy's not coming." Factual. Not loaded with emotional bias.
Use Legal Means to Keep Promises. You have a legal right for the commitments made in your divorce proceedings to be honored. Child Support can be collected for you. You don't need to burden your children with the fact that their daddy or mommy hasn't kept his/her commitment - again! Keep this kind of information to yourself and allow your children their concepts of their other parent until they have the maturity to understand the facts as they are if they are unpleasant.
I know a woman who's youngest son was six months old when she and his dad divorced. He spent the first sixteen years of his life cajoling her to get back together with his dad. He wanted an intact family. She did not want to mention his dad's homosexuality to her son, nor did she want to focus on his alcoholism and how it lead to verbal abuse when he was drinking. He loved his daddy and only saw that he was funny and took him to amusement parks frequently. He was his knight in shining armor. This lady worked to remain somewhat friendly with her son's father.
Once, when the boys had become young adults, she went on a camping trip where her ex got drunk and became verbally abusive, bringing up their old pre-divorce stuff. She did her best to calm him down, but both of their adult sons saw and heard the yuck. Her youngest son said "Mom, no one should ever be talked to that way." And she was finally able to tell him "This is the reason your dad and I could not remarry all those years when you wanted us to, son. I never wanted you to know about this."
Your children will look back when they become adults and be surprised at what you did not share with them because you cared enough to allow them to form whatever kind of relationship they could form with your ex despite all the nasty stuff you know about him. And your children will be grateful. Divorce and the shared custody problems isn't pleasant, but if you handle it correctly, maybe you can save yourself from that evil second dirovce.