I can't think of anything I really regret.
When we are growing up we all do silly things at times. I'm sure we've all had our hearts broken - maybe numerous times even, or told little porkies to Mum & Dad perhaps, skived off school occasionally, went scrumping etc etc, but none of those are serious enough to change my life or regret anything.
I firmly believe we need to experience tough things sometimes - as long as we learn from them.We would have a false impression of life if we sailed through it in fairyland, and never learn how to cope with both good and bad.
I would even say that, as long as we learn from a tough experiences, it can go a long way to understand others going through the same, as we might not have a clue about empathy otherwise.