I don't think that an adult should have regrets about his past actions. It's only normal to constantly recognise things in our past we feel we should've done either earlier or better. Sometimes, we can even recognise things we think we shouldn't have done. It's a natural occurrence that comes with growth.
What's even strange is to see someone without such a sense of introspection. It might mean that the person isn't growing. And a lack of growth is quite unhealthy in every sense.
After realising that you messed things up at an earlier age, the urge to regret is going to naturally come. I would've been a strong proponent of regrets if it had any form of potency to solve the underlying problem. But most times, it doesn't. If for anything, regrets will use up an enormous amount of energy and strength that would've been invested in a worthwhile adventure.
Everything in life is done with time and energy. What we spend our energy and time on matters a lot because both are limited resources. You wouldn't always have time; with every ticking hand of the clock, you're running out of time and energy. That's a scary fact. But it shouldn't scare you. It should rather make you accountable and responsible for how you spend your time and energy.
This daunting fact places a burden on all living beings to be mindful of what they spend their time and energy on.
Just like any resource, time and energy have to be invested, not just spent. We must use our energy and time to do things that will give us a return we'll be proud of.
If we spend today regretting our past actions, the truth is, we are likely to spend tomorrow regretting our today's actions. Regret isn't a good thing to spend our time on. It yields no substantial dividend.
No matter how you look at it, you did what you did with the knowledge, talents, skills, and abilities that were available to you at the time. Trust me, that's the best you could've done at the time. Now that you've grown, it's not fair to castigate or speak down on your younger self for doing the best it could.
The fact that you at least survived is a testament to the fact that you did amazingly well. There are people who ended it simply because they lacked the willpower to continue. That's one of the many amazing things you did. And if you look around, you're sure you'll find tonnes of other amazing stuff that your younger self accomplished.
They might not be big things. They don't even have to be. People don't have to clap for you to be validated. The fact that you're not getting the accolades you deserve doesn't mean that you're not doing amazing things.
Don't be ungrateful to your younger self. Despite its woes, it brought you this far.
When the urge to regret comes, take note of the things you didn't do so well and resolve within yourself to do better.
Spend today doing better than what you did yesterday. By doing so, your future self will be glad.