Apologise to someone after years but also THANK THEM?

I had a friend I was cruel to, just once, and for ten minutes of our lives, I was cruel to them. I said mean things. We were both young. It ended our friendship.

I was going places in my life at the moment. I was too busy to go back and apologise to them or think about them.

No, they're not my ex. We live in the same city. It's six years since then. I don't think about them often, maybe once a year when I see them walking down the street. We live in a big city (London).

In the moment I think I should go up to them and apologise.

But then something happens, my friend calls my name or my dog decides to chase a squirrel. I also think they might be more hurt if they saw me again.

I have their number. I have their address. I can write them a letter.

I don't just want to apologize to them. And I don't even care if they acknowledge my apology. I want them to know what a great person they are. They were extremely kind to me, even when I was cruel to them in my words. I am a better person because of them. They believed in my kindness before I did. I want them to know what a brilliant human they are. They have a heart of gold and I am upset with myself because I upset them for those ten minutes.

Even though our friendship was short lived, I want them to know that they are one in a million. And if they need anything, (a friend or financial help) then I'd love to help them. I don't expect anything in return. I just want to support this amazing human.

No I'm not at a low point in my life, infact life is amazing.

Is it okay to write them a letter or should I let this once-a-year thought go?