What's your biggest regret?

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Since everyone is a bit shy I'll start. The way I treated my mother when I was growing up is my deepest regret. I was watching a video who had the same issue and I resonated with it which is the cause for the question. I used to not think I was that bad growing up, but now my younger sister is treating my mother the same as, or worse, than I did growing up and it's made me rethink things. It's not fair or just how I treated her.
What's your biggest regret?
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