I've been having the same problem getting into fasting. I feel part of it is emotional. I do have some concerns about being able to keep going at work while fasting. There is also a fear issue, when I did a Master Cleanse fast lasting twelve days I plunged into a terrible deep depression, the idea of having that happen again scares me. I used to fast three days a month with no problems, it was easy and I looked forward to it. Now even one day is very hard. And, yes, the junk food craving have been very strong, its almost like my body is at war.
Sorry I have no advice for you, just understanding and compassion. However on the toothbrushing question. For many people the toothpaste stimulates the digestive enzymes in the mouth which "wake up" the digestive system in preparation for an incoming meal. It creates hungar and cravings. Baking soda with a small amount of salt is good for brushing your teeth, are just brush with plain water and rinse mouth very well. A tongue scraper is a worthwhile investment too. Frequent drinking of waterplus rinsing and spitting should help keep your breath bearable.