Strength of mind, and the will to persue your goals is what keeps you focused no matter how you stumble. Having set backs is a disheartening and demotivating factor that can make you feel like you you've just taken two steps back. However it takes a motivated person to brush off and move on and still try to reach that goal post.

What ever you're doing, trying to lose weight, studying for an exam, preparing for a company presentation, the principle is still the same.

I'm trying to shed a few pounds at the moment, and I admit the temptation to scoff food (especially scrumptious flirty food) when your belly is doing summersaults and lunch time is a long way away and all you've got to look forward to is a liquid meal replacement and a pot of yoghert, it's hard, it's frustrating, it's not nice.
I've heard a lot of talk about how diets should be fun or else they fail, advisors try to give you ideas about different foods and variety and a whole other load of stuff but I tell you, despite it all there is no joy in depriving yourself, especially when you are trying to break years of habit.
Here however is the key word.. 'Habit'
You gain habits, you break them and you make new ones.
Imagine a new recruit in the army, they have to shed every way of life that they know and recondition themselves to thinking, eating, living and being a fighting machine that is trained to kill if necessary. I know that that is a bit extreme compared to a simple diet but again the principle is the same.
You destroy your old habits, stop all that sugar, the treats that seems plenty and often, the desire to refill your plate or finish other peoples, the idea of diet break days where you eat what you can, cut the fizzy drinks and chocolate and crisps. It's hard, but only because a part of you doesn't want to change.
You know how it tastes, you like it, you know it's bad for you but still you want it. Thats the bad little gremlin on your shoulder tempting you. Tell him to do one!
Build new habits, they will soon become your way of life. You will no longer feel bad and accuse yourself of torture and denial.
Persevere... It will lead to success!