I have experienced cognitive dissonance when I have intended to go on a strict diet in order to lose weight before my beach trip and I'm cheating

I have experienced cognitive dissonance when I have intended to go on a strict diet in order to lose weight before my beach trip and I'm cheating myself by not adding chocolate to my calorie count when I am eating it. I keep making excuses telling myself I can have one cheat day a week but I'm not following that and I feel so guilty because I'm lieing to myself and I'm an honest person and I know it feels wrong.