Let's be honest, losing weight is never the easiest process. It takes some commitment and adherence.
It's also a journey of ups and downs and anyone who tells you it is easy or plain sailing is not giving you the full picture.
You will fall off the wagon, there should be no great shock in that but the trick is that instead of shrugging, sighing and packing it all in the best thing to do is recognise you slipped up and continue where you left off.
Don't go mad by trying to over-compensate by trying to starve or adding in extra activity. This really is more psychological based on guilt than -actually beneficial.
Take it on the chin and carry on with the diet.
Remember all diets work if you stick to them in the log run.