It is quite fine to have artificial sweeteners whilst going through the sugar withdrawal. Eventually you will minimize that as well, but initially even David Gillespie has the diet coke to help him through that initial period, so don't worry to much, some people just need more help to start with, dont be too hard on yourself, even if the process takes longer then the six to eight weeks, it really doesnt matter as long as you are aware and keep trying, you will get there.