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I didn't know it at the time. That's exactly what I think every Mother's Day now that my mom is gone.

What I used to think on this special Sunday was, "I should thank her. Really thank her. And I need to get her a card. I wonder if she'd want flowers. Maybe I'll take her to brunch." And I'd listen to all those typically sweet songs of motherhood, like "The Best Day," "Somebody's Hero," "In My Daughter's Eyes," "Don't Forget to Remember Me," "You're Gonna Miss This" and so on. But once your mom's gone, Mother's Days take on a very bittersweet patina.

One that, for me, is full of little regrets. So the song that now soundtracks my second Sunday in May? Tim McGraw's "I Didn't Know It at the Time." It was never released as a single, but it was love at first listen for me when his Southern Voice album came out in 2009. To the average listener, this song is not about mothers. But when I hear lines like, "Back when I had those fights with my old man, I never thought he'd be an old man" and "I never really thought that I was lost until I heard my dying mother sing 'Old Rugged Cross'" and "I didn't know all those years ago how the years would go flying by," I think about all the things I never got around to saying to my mom that I wish I had. The things you just know you'll have time to say later. But just like the song says, I thought I knew it all. But I was wrong. I just didn't know it at the time.