There are any number of reasons the yogurt could be making you feel worse. First of all, you are aware of the die off, right? You are SUPPOSED TO FEEL WORSE before you get better. The other possibilities are added Sugar in the yogurt, such as such as sucrose, a particularly deleterious preservative, or a reaction to the calcium. Candida loves alkaline minerals. As long as you aren't absorbing them properly (and if you have a bad case of candida, chances are you aren't), they make things easier on the candida and harder on the bifidobacteria and lactobacilli (the good bacteria) in your bowels. In fact, I read a study somewhere once that said that calcium is necessary for the yeast to maintain its fungal form. Of course, it's also possible that lactose DOES feed candida, and that these studies are wrong. But it doesn't seem like the kind of thing a professional mycology place would just get wrong. And I don't think I've read it wrong; it all seems pretty clearly laid out.
I can't say about Stevia since I haven't tried it, but I'm gonna try out other oligosaccharides that are supposed to help to promote the growth of beneficial bacteria in one's large bowel. Whereas a lot of things will just get absorbed in the small bowel before they can do you any good, the oligosaccharides can't be absorbed by the body. They are guaranteed to make it to your large bowel (which is candida HQ). If you can find one that is guaranteed not to feed the yeast (this Xylitol sounds pretty good), (theoretically) it will only be fermented by the good bacteria and will help you reclaim your large bowel. The probiotics aren't working, so I might as well try the prebiotics.