Weight loss Current Situation

I hate setbacks, Why do I forget that I worked so hard, to lose the weight? :(
I obviously, feel very sad, but I need to stop repeating this & take charge! 
So, I have been on track since a week,  My biggest challenge is to tame diet coke..I go without it, for 5 days and then I give up.
Highest weight

Highest weight  70.2 kilos (154.7 lbs)
Lowest weight   63   kilos (138.8 lbs)

Currently :         66.8 kilos (147.2lbs)

What do you think about setbacks? I read we can avoid them if we don't give up on ourselves!!!