Hi sarah. Your story is very common. I've completed a number of fasts and gone as long as 30 days but I still have the same trouble. It does require willpower and if I'm too stressed with my life in general it can just be too much for me to fast since fasting makes me attend those stressful feelings all that much more urgently.
Food the way we use it though is an addiction, that is the thing to remember. Breaking the 'habit' of eating too much too often is I think perhaps the very hardest of addictions to break also. The main reason is that social people seem to be 'feeders' and want us to eat at all times, so even if we cut ourselves off from advertising, shopping and food preparation unless we become hermits for the duration of our fasts it's very hard to avoid being offered, sometimes constantly, all types of different foods, some of which are bound to tempt us. The addiction is constant just like the habit of picking up a cigarette, it reminds us when our mealtimes were even when we're not consciously thinking about it. Our senses intrude on us with memories of food from smells and sights, sounds and textures. And our thoughts are our worst enemies most of the time, coming up with all sorts of excuses to eat- most of which won't pass inspection after the first bite!
So you're not alone by any means Sarah. Stay strong if you can! It's only as bad as all that when we think we've failed really. For most of us it's much easier once we break past those initial cravings. It has helped me to allow for 'failures' too. To focus on the whole task of the fasting experience rather than just the days not eating. So if I start a fast I always prepare myself intellectually for the day I'm going to break it and how I'm going to break it. I don't set a time limit for that and whether it's 1 day, 10 days or 40 days makes no difference, if I can break a fast correctly then I have succeeded in my fast not failed in any way.
Good luck. :)