Is it true that you don't really know the person until after you break up?

I dated a girl once, then found out after the fact from a 9 year old of all people that this is what she does. Stays with a guy for 2 or 3 months, gets bored then moves onto another guy. Rinse, wash, repeat. The 9 year old didn't say it like that, but I kind of put two and two together. She asked about us and I said she broke up with me and the 9 year old goes AGAIN? So I figured she does this with a lot of guys. Anyways, it's like how come when you're with someone, everyone keeps their mouth shut? That would have been nice to know in the beginning

Is it true that you don't really know the person until after you break up?
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