For the “Bitcoin is money, there's nothing to be regretted” replies, I have nothing to say, but this. You've confused or haven't understood the definition of the verb “regret”. When you're the owner of something, whether that's Bitcoin or a baseball card, and years after you've got rid of it suddenly costs much more than the price you sold it, you should feel regretted. You should feel contrite about not owning it anymore, because it was down to your decision.
The fact that Bitcoin is money says nothing; you saw the guy who paid a fortune for two pizzas and it was down to you to not repeat the same mistake. Unless if you believe you're the one who'll do the first step(s); if that's so, then good luck.
Was it a mistake, though? I hope not. If it was, then it is really sad. Sadness over what's done in the past is regret enough.
Personally, however, I don't regret much. I don't dwell much in the past. But I definitely agree with you that there is such a thing as regret. It is easy to say, "I did what should be done with Bitcoin, anyway." But we are human beings; our rational minds know how to compute. We have wishes. We have wants. We have dreams. And if we are to think of more than half a billion USD in today's price spent on a couple of pizza a little more than a decade ago, it is normal to feel sad of what's missed.
Of course, no point crying over spilled milk. After all, life moves forward not backward.