That unfinished sentence still haunts you, and every waking hour (and for you, there are many) you spend searching for a way to finish it. That was a distraction for you, anyway, to draw your attention away from the fact that there are other things you are forgetting that you would much prefer to remember.
For example, you can remember the agonised expression on her face when she realised that you couldn't come for her. That she really was stuck there.
You can remember that with a painful clarity, but have forgotten the exact feel of her lips under yours.