Usually disease does not get worse if you are treating it by means of feeding properly or even external means. But during a fast it may get worse before it gets better. Our body chooses any means to get rid of toxins including any existing conditons and expecially the skin.
Yes, you wll get better with fasting. Make sure you rest, keep warm and drink water by thirst, hopefully, distilled.
After your fast is completed you should only eat raw fruits and vegetables and then after a few weeks do a Liver Flush. Liver Cleansing should elimiinate Psorisis. If it does not two weeks later another Liver-Flush would be necessary.