Happens to me too often. Usually when I start up it is no longer an issue. On the other hand, I lift at home, so when I'm really in the dumps, a few dozen steps and I'm taking a shower and in bed... Quiting on crappy days is much harder to overcome.
I bit the bullet most of the time, when I don't it usually means pushing the session off to the next morning. It extends the week a little, but doesn't end up interfering with other sessions. Plus if I'm that low mentally in regards to lifting, an adittional 12 hours of rest is usually enough to get my head back in the game.
As for missing specific sessions, happens very infrequently for me, but when it does, it doesn't bother me to much. That lends itself to the "mental" recovery and also, I train 20 days a month and move as much weight in one day as I did in one week not to long ago. Missing one or even two day's isn't going to mean much in the long term.