Okay -
First - 2 pieces of pizza and 1 breadstick is nothing to feel guilty about. It may be offplan, but it does not qualify as a tremendous cheat like an entire cheesecake (ahem).
Second - Our culture uses food to celebrate. It is very difficult to break the mental pattern that links happiness with yummy food. Although I try very hard, I still do it occasionally (very fancy dinner at a very fancy restaurant for my birthday, enjoying local delicacies on my last trip to Singapore). All our lives we're going to be faced with co-workers that bring in birthday cakes, spouses that want to take us out for anniversary dinners, nachos at the state fair and wanting to eat ice cream when we've had a bad day. Definitely impossible to be perfect all the time
It would be very difficult to spend an entire lifetime "on plan." A goal of 80/20 is not a bad goal and 2 pieces of pizza and a breadstick after a good interview definitely fits into an 80/20 lifestyle. I did find that I was more likely to be tempted by "bad" food if I were overly restricting calories. If I were depriving myself, the idea of something sugary/greasy was more attractive (since the body craves quick energy in the form of fat/sugar). If you are frequently tempted and can't resist, maybe look at what you're eating every day to make sure the body is completely satisfied with what you are eating (calories, healthy fats).
If you feel really badly about it - try to change how you reward yourself. Pizza is good but a pedicure would be good too

(or a cute new pair of summer sandals).
Don't feel badly, don't beat yourself up. Have a nice dinner, drink lots of water and keep going! You are doing great.