By Zahna Eklund Losing weight takes a lot of willpower – and it turns out we’re holding ourselves back with one common mistake. If you’re trying to lose weight or have tried in the past, you’ll know just how gruelling the process can be. You have to have an iron will to stay away from tempting takeaways and sweet treats, and reading nutritional information while food shopping can quickly become depressing when you realise just how much sugar and fat is in some of your favourite snacks. But according to health experts, there’s actually a common mistake many of us make that stops us from achievi…