This is a great quote by Mark Twain. Generally speaking I think he's correct. We all succumb to thinking, from time to time, about what could have been only if we did.
But, we also all have things we wish we didn't do. For all of those things - like flashing your boobs to a freeway of drivers - that we did and might now regret I simply hope you did them with a girlfriend so at least you have someone to laugh at your stupidity with. Flashing your tits to speeding traffic... these girls are going to cause an accident.
I think it might be better to be more disappointed by what you didn't do that by what you did - hopefully you remember why you didn't and hopefully there was a good reason.