So I made a bad choice at lunch, maybe had too many calories? Tough luck, kid, but don't think it has "ruined" your day. Take a walk or do some jumping jacks and make a better choice the next meal. You can't punish yourself forever. Accept it and do better next time.
If you treat your new lifestyle like a prison sentence, it will start to feel like one very quickly. You will be miserable and you will fight yourself every step of the way. You might fail sometimes, but that doesn't make YOU a failure.