When I was a teen, I did something absolutely despicable. I went before a judge for my crimes, and I did some time and a whole lot of therapy and I definitely became a better person for it. It does not at all absolve me or excuse what I did.
To my teen crush: I'm somewhat sorry for not approaching you, but you gotta understand it's not easy for a boy in this society to make any advances at all.
When it comes to the ones still living, I simply tell them. For example, telling my mum that I love her.
Except maybe my ex-fiancee. She did affect me more than I'd like to admit, but frankly? I don't want any contact with her, and I already told her everything that I had to.
For the deceased:
relative 1 - I regret not spending more time with you. Drinking yerba, chatting about random stuff. Also I miss the cheesy polenta that you prepared.
relative 2 - if you were alive, I'd probably want to get drunk with you once. (Your daughter would get mad at both of us.)
relative 3 - I should've bought fireworks when you kicked the bucket.
friend - dude you're THE major reason I like and enjoy music nowadays. You were always selling yourself short in this matter, but listening to you talking about songs from all the world was really great.