It happens- you either get through or you don't. I've failed MANY times and honestly am surprised I've made it past Day 1 (on Day 2 now). I swear I think I've attempted 20 fasts and have only gone through 3 (10 days)- the odds are seriously against me/us.
It's a shot of the dice, really, all depending on your mood, your environment, etc. I can just, in the split of a second, go eat my face out...and I keep my fingers crossed all damn day that I won't. :p