I’ve mentioned this before but my biggest regret is that it took 40 of our 50 years of marriage before we both started talking openly and honestly about what we really wanted and needed sexually.
When we did finally talk it elevated our relationship to a new level.
When I was younger and less confident I am sure I missed out. But I don’t have any regrets over it.
That’s the story of my life too, and it eats away at me now and again in low moments.
I’m happy now with my current partner after 20+ years of course, and also the prior long term relationship wasn’t the worst either, but I do look back with regret at those ‘wasted years’ of my youth.
I think of it more like “unrequited love” I have a few of those. It’s sort of bitter sweet isn’t it.
I lived in San Fran for a bit many years ago. I saw a pretty tasteful advert on Craigslist for a ‘free-use’ housekeeper for 3 women and two men. Bottled it, but it’s always been a big ‘I wonder’ moment for me
When I was in high school, there was a girl who sat next to me who I liked a LOT. Several times when we were taking tests, and I was struggling to find answers, she would notice, and would give me a glimpse of the formulas she had written on her thighs above her ( usually ) short skirt. I never had the nerve to ask her out, or get more social with her. There was a day when she and some of her girlfriends were skipping class and heading to the beach, and she implied I should tag along. My folks were very strict, and me skipping a class would not have been acceptable, so I declined.
Maybe nothing would have come of it, but I severely regretted not taking that chance. I did have many pleasurable fantasies about the possibilities though.
Was my 21st (far too many years ago!) obviously very drunk and at the end of the night I’m back at the student house with my (at the time) girlfriend and one of our (female) housemates. Unusually, I ended up in the housemates room with the girlfriend and I just wanted to go to sleep, so I did.
Was only told weeks later that they planned a threesome as a treat for my birthday! 
Tried suggesting we do it, but no joy 
Sometimes the moment is lost… Carpe Diem !