Having One Bad Meal Doesn’t Mean You Should Start Your Diet Again "On Monday" (and Why)

Have you ever eaten (or drank) something you "shouldn’t" have done while trying to lose weight? Or even had a whole day go completely tits up and that led to you thinking (or saying):
"I’ll start again on Monday!"
(I swear, it could be Tuesday and a large number of people would still want to start ‘next Monday’)
This is the outcome occurs because you believe you’ve ruined the whole week and therefore feel like there isn’t any point in carrying on.
This is simply not true.
Why?
(For those who’d rather watch a training video you can click the pic below)