You haven't ruined yourself. You haven't ruined anything. In all probability , virtually all of your gain was just water retention (not fat gain, just temporary water retention.) However, you're not just dealing with carb water retention, you're also likely dealing with sodium-related water retention).
You're likely to encounter water retention periodically, even if you never "cheat" (I don't like that word because it sets you up to see it as bad, sinful, naughty - at any rate, blaming and judging yourself harshly for a normal, and sometimes even unavoidable occurrence).
In my experience, it can take up to 10 days for my body to deal with water retention. 10 days has been my personal maximum, and that's usually involving TOM issues. Carb-only water retention for me tends to last 3 to 5 days. Sodium-related water retention I can usually get rid of in 2 days, if I drink a few extra glasses of water during those two days. Any combination of water-rentention variables (TOM, exercise, increased sodium or carb levels, illness...) tends to prolong the water retention, but I've never had it last more than 10 days for me, but I've met people in weight loss groups who've had more prolonged issues (many of those people were older or had heart or vascular problems or were not on low-carb diets).
Regardless, the weight loss will resume - whether it takes one more day or 14. Be patient, drink a little extra water (don't aim for more than a gallon a day without talking to your doctor).
Keep this in perspective - You didn't murder anyone, you didn't steal candy from a baby, you ate some pizza - hardly a capital offense. Hating yourself only adds stress and all those nasty stress hormones that increase hunger and slow metabolism, making weight loss even harder.
I do best on a low-carb and no wheat diet. All sorts of nasty things happen to me when I don 't eat low-carb. Really nasty things. Nastier even than weight gain, but hating myself for making mistakes has never helped me make fewer mistakes. It doesn't work, and it makes me unhappy. It's a no-win situation.
I make a lot of mistakes, but I don't let myself stress over the mistakes anymore. I learned that hating or beating on myself only makes me less likely to learn from the mistake. No amount of self-punishment has ever led me to becoming a better person - only a more neurotic one.
We're taught to hate ourselves - and we're taught to fail - and we do it because we see everyone else doing it. We would never slip during mountain climbing and decide "Well that little slip ruined everything. I might as well just throw myself to the bottom because I'm such an idiot (and so I can start fresh tomorrow)."
We do that with weight loss, because it's how weight loss tends to be done. We've learned an ineffective pattern, but we do it anyway, because we don't have many models for doing it any other way.
Learn other ways, but don't hate yourself for making mistakes. That doesn't help you stop making mistakes. Instead you'll feel like a worthless idiot - and when you feel like a worthless idiot, you'll act like a worthless idiot. People tend to meet their own poor expectations of themselves.
You're smart. You're creative. You can do this, and you deserve to. It's not about being perfect 100% of the time, it's making the best choices you can, and not letting one less-than-perfect choice become a pattern of bad choices.