Listen, you had a splurge, it is over with so pick yourself up and get back to it. It happens to every person that posts here, usually lots of times. You cannot get discouraged when this happens, you just have to chalk it up and move on.
Absolutely - I've done it more than once. Draw a line underneath it and look forward. Get straight back to healthy eating and maybe go out for a long walk to balance things out a little. Not worth stressing over
Sometimes free food events happen and we eat more than expected. Its not as bad as you really think, since you have to eat calories daily anyway. Lets say you need 500 calories a meal, and ate 850. You're really only 350 over for the day.
And don't freak out too much. It'll make dieting seem much harder than it is psychologically.