The partying and pizza relapse is a classic. Don't worry about it. If it's mostly a one-off thing it doesn't count. Just get back on the ... uh, hobby horse?
The first year when I was vegetarian I got invited to someone's house. I thought they knew I was vegetarian, and maybe they did know, just that they didn't know what 'vegetarian' meant? So they served fish. And I was too polite to make a fuss about it. I wasn't happy with myself afterwards, but I decided to avoid any further such misunderstandings or awkward social situations for the future. And honestly, things have become a lot easier over the years.