A lost love. Deny it who will, ridicule it, treat it as mere imagination and sentiment, the thing is and will be; and women do suffer therefrom, in all its infinite varieties: loss by death, by faithlessness or unworthiness, and by mistaken or unrequited affection.

Nothing is more excruciating than hopelessly longing for lost love.

It's the tragedy of loving, you can't love anything more than something you miss.

A mighty pain to love it is,
And 't is a pain that pain to miss;
But of all pains, the greatest pain
It is to love, but love in vain.

It's the tragedy of loving, you can't love anything more than something you miss. (p. 208)

It is sad to love and be unloved, but sadder still to be unable to love.

Every time we make the decision to love someone, we open ourselves to great suffering, because those we most love cause us not only great joy but also great pain. The greatest pain comes from leaving. When the child leaves home, when the husband or wife leaves for a long period of time or for good, when the beloved friend departs to another country or dies … the pain of the leaving can tear us apart.
Still, if we want to avoid the suffering of leaving, we will never experience the joy of loving. And love is stronger than fear, life stronger than death, hope stronger than despair. We have to trust that the risk of loving is always worth taking.

The hard part of loving is that one has to learn so often to let go of those we love, so they can do things, so they can grow, so they can return to us with an even richer, deeper love.

Loss is the cost of loving

Of all pains, the greatest pain
Is to love, and love in vain.

Lost love is still love.

To love is to suffer and there can be no love otherwise.

To love, to be beloved again, and know
A gulf between us:—aye, 'tis misery!
This agony of passion, this wild faith,
Whose constancy is fruitless, yet is kept
Inviolate:—to feel that all life's hope,
And light, and treasure, clings to one from whom
Our wayward doom divides us. Better far
To weep o'er treachery or broken vows,—
For time may teach their worthlessness:—or pine
With unrequited love;—there is a pride
In the fond sacrifice—the cheek may lose
Its summer crimson; but at least the rose
Has withered secretly—at least, the heart
That has been victim to its tenderness,
Has sighed unechoed by some one as true,
As wretched as itself.

The third answer is the most difficult one to grasp, but it's also the one that can save you. The love you lost, or the love you wished for and never had: That love exists eternally. It shifts its shape, but it's always there. The task is to recognize it in its new form.