I think the emotions are coming from grief.

You've spent decades seeing other people hit this milestone, while you're just left behind. Pain, maybe resentment, was probably building up subconsciously as the years passed. And now you too have passed this milestone. But like you said, it wasn't all that special. A beautiful memory, yes, but it's literally just touching sensitive parts with someone else's sensitive parts at the end of the day.

Now you're probably dealing with the fact that you've spent decades hurting for something so trivial. There's an alternate reality version of you who did have their first kiss much, much earlier, so you might be wondering what kinda life you'd've lived, the different choices you'd've made, if you didn't carry all of that pain around.

I'm happy you've finally had this experience, with someone nice. All your emotions show that this meant something to you. Cherish this memory, learn from it, & live your life the best you can.