The main hurdle for me in trying to eat healthily is the feeling that I am always missing out. Intellectually, we all know the benefits of eating well, but this is not enough, particularly if, rather than making you feel worse, sugary and fatty foods actually (temporarily) boost your mood.
Again, The Places That Scare You ('TPTSY') explains that this is normal - it is human nature to mistake dissatisfaction for happiness. I was struggling with how to overcome this when I read Chapter 4 of TPTSY, which covers the benefits of meditation. One of the benefits is "clear seeing" (or dare I suggest - "seeing clearly") - we begin to see bad habits as restrictions.
When I read this I realised that I shouldn't regard Tim Tams as standard benefits that everyone else in the world gets to enjoy (because of course when you are thinking this way you conveniently ignore diabetics, the third world, kidnapped children in the Ivory Coast forced to harvest cacao from sunrise to setset, etc), instead they are barriers to my true enjoyment of life. Since then I have found it far easier to say no. For example, I said no to a Tim Tam just three hours ago and didn't feel deprived.