It's much more painful for me after having invested my love into a relationship-when it ends up being temporary for the other person.
Having her not in my life leaves an empty place. It's been.... fucking almost 20 years. It's not a new loss. But the emptiness, it's still there.
Much less empty (not necessarily less painful) if the person with whom I get involved remains a significant part of my life. Which I think is more of what MG is trying to get to.