1.5lbs = 5250 kcals. I VERY much doubt you ate an EXTRA 5250 kcals. It's most likely water retention, especially if you ate processed food like bread/pizza/chips/cakes.
Drinking water is a good idea, also not weighing every day is even better - your body naturally fluctuates over a pound or two over the week.
You haven't let yourself down, some plans allow a "free day" once a week so take it as a free day, eat healthily tomorrow and it will all balance out. It is what we consistently do that produces results. Heard of the 80/20 rule? If you eat healthily 80% of the time, that's what produces results, if you splurge 20% of the time, then it will still work out ok.
We are in this for the long haul and there will always be slips and doughnuts that you just can't resist. We're HUMAN not some machine that should ONLY live on salad and fruit
Give yourself some slack and get back to it.