I can relate to your question and the guilt can sometimes eat at you, which is a good thing, because that means you have formed a habit to get to the gym and work out, but it's ok if one misses a workout every once in awhile, just don't let it slide and lose the habit.
To be honest, your day off is like your muscles saying thanks. Because muscle growth takes place when you are relaxed and resting....not when you're in the gym. When you miss a day, I suggest doing some situps, pushups, and side bridges before you go to bed. That 10-15 minutes to do those three exercises alone will make up for you time that you missed in the gym. Plus, it will give an awesome pump before you fall asleep.