We get stuck there, too, forever the victim, the have-not in the realm of love.
Strange as it may seem, a grudge is a kind of clinging, a way of not separating, and when we hold a grudge, we are clinging not just to the person, but more specifically to the bad part of that person.
It's as if we don't want to live our lives until we have this resolved and feel the security of their unconditional love. We do so for good reasons psychologically.
But the result is just the opposite: We stay locked into the badness and we don't grow up.
So let go of resentment and grow up!