Co-parenting is one of the hardest things to do. Your marriage already broke down, and now you have to work together with your ex-spouse to parent your child. While it may be difficult at first, co-parenting your child is usually the best thing to do in the long run.
When you are co-parenting, your child benefits from you and your ex getting along (or at least appearing that way) and working together. It shows you're committed to making your child's life easier and committed to raising him or her as a team.