Forgive and forget? Why forgiveness is not always necessary

There's so much talk about forgiveness these days, with some suggesting that, without forgiveness, a relationship doesn't have a future.
I beg to differ.
Forgiveness means different things to different people. What it doesn't suggest on any level is that the betrayal was "okay". Janis Abrams Spring, author of How Can I Forgive, calls that "cheap forgiveness."
For me, forgiveness means letting go of resentment...but it also means a little bit more than that. Something I just can't quite define, but perhaps resembles a lifting of the spirit.
I have days – most days, in fact – when I'm sure that I've let go of all resentment. But now and again, something will happen – say, my husband forgets to pick up my favorite ice cream flavor, but remembers to buy his own fave. In an instant, my eyes narrow, my lips purse and a nasty little voice hisses something along the lines of "it's not enough that he cheated, exposed you to god-knows-what diseases and lied about it but HE CAN'T EVEN REMEMBER TO BUY YOUR FAVORITE ICE CREAM, that bastard!" And I wonder if I know anything about forgiveness.
For now, I'll settle for acceptance. That my past is immutable, though I can learn from it. That my present is unfolding minute by minute. And that I can create a future in which I just might grasp what genuine forgiveness truly is.