Just my 2 cents.. Still new at this myself and everybody is biochemically different. However, the sentiment "felt horrible afterwards, nauseated but not able to vomitt" you expressed , I believe is probably from breaking the fast inappropriately. I am guessing you did not break it with juice or juicy fruits?
15 days is a good fast and your digestive enzymes and the like had already entered into the resting state while ketosis took over. You may have felt moody for a variety of reasons, I know quite well the fasting symptoms are not always pleasant and pushing through it when you are drop dead can be hard too.
When you ate that "something", it was likely enough to reverse you back to glucose metabolism from the ketosis state. However, since you went back to Water Fasting immediately, it is likely to take you less time to reach ketosis since your liver is not packed with glycogen stores and you only ate a small amount.
I don't think you should feel that you ruined everything. 15 days is an accomplishment. Even if you had not broken your fast, there is no guarantee your body will not plateau at a certain stage to do other forms of housecleaning or if would rumble on down to your set goal. I find that I cannot predict how much I will lose nor always how long I can physically stay with it, as your electrolytes , possible stressors in your life, activity level and other events in your life might decide that for you . Just something to consider.
First couple of days you are switching your metabolism back to ketosis, so you probably briefly pulled yourself out . You might want to consider consuming vegetable juices as a means of dropping weight, that would eliminate your weakness and perhaps enhance your mood. It could also have been related to detox issue and in that case you just push through it. The important point I learned from the first fast is , the refeeding part is most vital. It will spare you the discomfort you felt after you ate, but it is also a must, as you have to wake those resting enzymes back to normal activity through the use of gentle juicing and high water content frutis and vegetables .
See Fonty and Chrisb1 posts. They offer a wealth of information to guide you .