Maybe you could try one of those Lean Cuisine pizzas I keep seeing on the telly - would love to hear what they are like. I think Weight Watchers do pizzas as well.
Perhaps you could think of ways to avoid it happening again or somehow incorporate a reduced portion into your eating plan - I mean lets face it every now and then its great to have a takeaway without thinking about it, you know, just for taste sake or to be like everyone else. Maybe 2 slices and whip a side salad - pizza without the side of guilt???
You hit the nail on the head though, tomorrow is another day, its not what you've done but what you'll do that matters.