If you lose your way on your way to a particular place, you will obviously ask for directions, backtrack and then be on your way again – the same should be the case with healthy eating – the occasional indulgence is fine, so long as you keep to healthy pattern most of the time.
We constantly have to make choices, and so long we make healthy food choices by and large, our health ought to be good enough to take the assault from the occasional unhealthy component in a meal.
The point is that your health should be good enough in the first place to be able to sustain itself even in the face of an unhealthy indulgence; not that there should be no unhealthy indulgences ever – this is neither reasonable nor expected given our lifestyles.
Also consider what fad diets do your system – radical calorie restrictions such as fasting or cutting out entire food groups are bad for the body. They weaken the body and are not good in the long term.
For this reason, rather than a diet that has a finite duration and which will ‘end’ after a period, healthy eating has to be life long effort, an ongoing process with no real end in sight.