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After a divorce, it's a given that your ex will still be in the picture - your children's birthdays, weddings etc. You'll need to accept the fact that this is a new reality. Be sure to deal constructively with your ex when events together cannot be avoided.
The two most important sentences you'll ever learn are, "I made a mistake", and "I was wrong". Healthy families don't require perfection, but they do require the willingness to admit when they do wrong.