Ruined my day because of one chocolatebar?

I actually ate 961 cals then I ate one small chocolate bar and thought its 200 calories but turned out it's almost 500 calories. Now I almost ate 1500 calories. Will I gain weight? I always feel like my stomach is really full which makes me think I counted my calories wrong. Is that normal?
Ruined my day because of one chocolatebar?
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