I don't have any true 'regrets', because I don't see the point in getting upset over something that can't be undone, no matter how riled up I get at them.
That's not to say I haven't made my own fair share of mistakes in the past. I have, and I never forget them to make sure that I don't repeat them. I am who I am because of those mistakes.
My biggest mistake would probably be going completely bonkers over some girl in 7th grade. I became completely unhealthily obsessed with her, texting her quite literally every thirty minutes, even if she didn't respond. When she did respond every once in awhile, I would just make up a fake sob story, basically fishing for compliments. Whenever she'd compliment me, I'd send the most cheesy texts to her, essentially praising her for being the best thing ever since slice bread. I'd do this so frequently that her friends started noticing my texts, and then started teasing me about it. Shortly afterwards the teasing started, she started to ignore my texts entirely. I eventually came to my senses, and realized how much of an idiot I was being, so I just told her that I was genuinely sorry for spamming her phone and promised to never do it again. She forgave me, but I still fear talking to her.
To this day, I can't bring myself to look at her or her friends in the eyes because those memories still haunt me.
I'm glad it happened, though, because now I know not to go bananas whenever a girl gives me her phone number and not to become a stalker.