There's 8 billion people in this world... Why?

This is my first time posting, but been a lurker for a while. I've made the odd comment here and there trying to help people based on my experience, but this is the first time I feel I've needed help, hence my first post.

Long story, I've wanted to put my entire story here for some time but honestly, didn't want to write it all out, and I didn't think I needed the support. I'm a bottle of wine deep at the moment and feel like I need help.

In a nutshell, wife had a drunk ONS before we were married, while we were engaged. I didn't find out for 5 years after, after marriage and 2 kids. I always suspected, but she vehemently denied. She came clean after those 5 years. It took some time, but we reconciled. It's now been 10 years since DD, 15 years since it happened. We added another kid (3 total), and my life is great. I couldn't be happier. 100% trust, just a fantastic life. Successful reconciliation story.

I live in a completely different city from where this all took place, 15 years ago. Tonight I took one of my kids to their sport. Watched the game. After, I was walking to meet them after it was over. Was walking through a door, and someone held it open for me. They looked at me like they recognized me. I have terrible facial blindness, something that has bothered me for years. I can't remember anyone's face or name. They said my name, in a questioning tone. Ive been working on this fault of mine for a while now, and normally I would have just said hi and kept walking. But I've decided recently to put in more effort. I looked them straight in the eye and said 'im sorry, what's your name again', thinking they were one of my kids friends parents. He said his name, and the recognition came flooding in. It was like I was punched in the chest. My heart started pounding so hard and so fast, I thought I was having a heart attack. I then had my first out of body experience. I don't remember what was said, or what I did. All I remember was shaking his hand (I know!) and asking if he lived in my city (thank God he doesn't). After a few words, I said my kid was waiting for me and left.

Since then, I came home, started drinking, and can't for the life of me get my heart to stop racing and pounding! I feel like this has all been a dream. I can't believe, 8 billion people in this world, how the hell do I run into him, randomly, in my city!

Why the hell did I shake his hand! Why was I so cordial? I'm disgusted with my self! Why didn't I tell him how I really felt? I'm not a violent guy, never been in a fight or thrown a punch in my life, was never going to do that. But why did I engage him. How do I get my heart to stop racing? How do I get to sleep tonight! I honestly never thought I'd run into him, I never even thought to prepare for this, never thought it would happen.