Loving Your Enemies in the Old Testament

Here's a powerful example from the Old Testament of what it means to love your enemies.

Ruthless witnesses come forward;
they question me on things I know nothing about.
They repay me evil for good
and leave me like one bereaved.
Yet when they were ill, I put on sackcloth
and humbled myself with fasting.
When my prayers returned to me unanswered,
I went about mourning
as though for my friend or brother.
I bowed my head in grief
as though weeping for my mother.
But when I stumbled, they gathered in glee;
assailants gathered against me without my knowledge.
They slandered me without ceasing.

Psalm 35:11-15, TNIV

(The Psalmist then goes on to pray for God to vindicate him in the usual OT way.) But what really struck me was this as an example of loving enemies. What does it mean to love our enemies? To behave like this to them, even if we knew they would turn on us. And to do it again after they have...