Divorce does not have to take you by surprise. In many cases, you can see the red flags from a mile away. You have been dealing with these issues in your marriage all along. They finally got to the point that they caused you to split up. That doesn't mean they're new.
When you and your ex got divorced, you got full custody of your kids. Your ex is simply not reliable enough. While they still get to come visit on the weekends, most often for supervised visitations, you are essentially raising your children on your own.