That Thing (I'm Sorry)

"I'm sorry" I said. Too little, too late? Perhaps, but it was the truth and needed to be said. I looked at her as she sat by the window.
"Sorry for what?" She didn't lift her head or look at me.
"For never....." I began. The words jack-knifed in my throat. I looked at her, the back of her neck, her hair in ringlets, a picture of benign indifference. I shuffled forward, sitting at her feet. I stared at her immaculate ankles, the ankles I used to kiss, that used to bang into my feet every night as she wriggled to get comfortable. I missed those ankles. "I'm sorry. For not saying that thing"
She turned to me with a curious glint that I couldn't help but gaze at. "That thing you wanted me to say. That thing I left always unsaid. That thing you used to whisper into my ear when you thought I was asleep. That thing you wrote in chocolate sauce across my belly. That thing that was always on the tip of my tongue. That thing that could've saved us. Maybe not."
A tear drop clung to her eyelashes. She looked at me for somewhere between five seconds and six hours, I can't be sure, then smiled and said "Say it now".
"I can't."
"Why not?"
"I don't know how to."