I agree with ED, you should accept that you needed that bit of a nibble and that it could have been so much worse, but you kept it under control which is something you may not have been able to have done in the past. If you make it part of your diet, something you needed to do to get through the day, and dont feel guilty about it or make into this big bad thing, then it's so much better.
I could have had a panic about all the sushi I ate on Friday night but decided that I wasn't going to do that. I ate it, enjoyed it, and put it behind me, and even better, it didnt affect my weight loss at all (although I still think that was a fluke). If I'd really worried about it and got myself worked up about having "ruined" the diet, felt like I'd cheated, or any of the things I'd have said to myself about it in the past, it may even have affected the numbers on the scales. I do think that some of this is psychological, how we feel can affect what happens when we stand on the scales.