"Experience is the worst teacher, it gives the test before teaching the lesson." I put that in quotes because it's not mine, but I'm not sure who said it and I am sure that I don't have the quote exactly right. I am sure that I have the meaning spot on. This has to do with why those of us in the second half of our lives are constantly lamenting that we didn't know what we know now when we were younger.
Imagine going back to high school or college with the life experience and knowledge we now possess. Sure we'd make mistakes, but not as many and all while clearer on possible consequences and the general direction we wanted to go.
And yes, dating and relationships are a large part of this lament. No matter how happily married we are there are memories of those "could have been" relationships that haunt us.
Even so, no matter who old we are it's never too late to learn, grow, evolve, change, be an old dog learning new tricks.
We can ruminate about our pasts or we can secure our futures. I learned that from experience.