I wouldnt drink anything with artificial sweetners especially aspartamene or any of that stuff with aditives, e numbers whatever...dont want to preach but they are really unhealthy. We may be fasting largely for weightloss but we want health benefits from fasting too otherwise it'd be counter productive. There must be substitute treats you like just as much that could be included into your fast, try juicing
fresh apricots with apple it'll knock your socks off! I think fizzy stuff is pretty dehydrating too, not what you need. Good luck, Emma.