There is hope - please do not give up! I am happy to see you are looking for ways to rectify this condition. That is great
Has your vet taken xray's of your kitties colon? Has he said it looks streched/enlarged? Has he mentioned Feline megacolon? My cat had the same problems as your kitty is having. At first she was diagnosed with constipation but it then progressed into megacolon. Is the vet porscribed stool softner called lactulose? My cat was on this as well and it STOPPED working for her. I use a stool softner now called Miralax that is amazing. With megacolon it is most likely a motility issue. The cats muscles in the colon can not push the stool out on their own (because the colon is really stretched). So a motility drug is usually required along with Miralax. I use Miralax & Cisapride on a daily basis. Does not mean your kitty needs both but I want you to know there is hope. All your kitty may need is the Miralax. You can buy Miralax OTC at a pharmacy or off the shelf in the medication section at Walmart. It is a tastelss/odourless powder that can be added to your kitties canned food (with some water).
Another thing is you may want to try a low fibre food grain free canned food (MAX fibre content of 1 to 1.5%). Most dry foods have a higher fibre content. Fibre does not do well with some kitties (especially severley constipated / megacolon kitties) because it bulks up the stool making it even harder to pass. And the kitties are already having a hard time passing stools.
I belong on a group on yahoo "Feline Megacolon" that deals with constipation and megacolon in felines http://groups.yahoo.com/group/Feline_Megacolon/
Come on over and join for some support and advice. There is alot of info in the files and archives for you to look over and take to your vet if he is not versed in megacolon.