Trying to understand the reason why you broke up
It seems that your partner hasn’t told you something and you think that if you could just discuss it, the situation could be fixed. But there is a high probability that you will spend several months trying to find out what was wrong.
Why it’s harmful: It is really unlikely that you will manage to convince your partner to change their mind. If the reason for breaking up wasn’t some terrible mistake that you made and have to apologize for, it’s no use trying to fix anything.