While ending the relationship, if someone grieves that they wasted their time.

That validity of that statement depends on your attitude. No time is wasted unless you choose to waste it. If you went through a relationship and never learned a thing or lived a single moment of it then that's on you. The final outcome doesn't render it a waste of time. We are not blessed with the ability to see into the future and hence our investments won't always bear fruit, but the experience itself gives you something.