Why does it still hurt so much?

I recently dealt with a situation where someone about whom I care a great deal caused me a great deal of pain. It's supposed to be all better now, and for the most part it is. But every now and then I still get this twinge inside, like someone's twisting my guts into a knot. Don't get me wrong, I've completely forgiven the person in question. I'm not even a little angry anymore. I'm just...sad. Very, very sad. And I wish I could just turn that off.