I have no idea how many calories, fat etc. are in that sandwich, but I know that you are not horrible for having eaten it!

The only thing that could 'blow your diet' if if you were to decide that it has, and would consequently make wrong choices for the rest of the day, the week, the month... And so on.
It's hard to do it 'the perfect way' from the start, if it's ever possible to be 'perfect' at this (yeah, right). Trial & error as well as little bumps is something most of us have/have had to go through at some point. If this wasn't supposed to be your last meal of the day, just make sure that tonight's dinner will contain veggies, lean meat, in a word: healthy choices, and tomorrow it'll be behind.
