First off, get some no-sting barrier wipes for when you change your wafer again. They help clean the area without burning and they help protect the skin.
Second, it sounds like your wafer is a little too big (its hard to get the exact correct size most of the time, and your stoma may have shrunk some more) or not holding up as long as you're wearing it because stool eats away at it. If you know it's the exact correct size, maybe change it a day earlier next time.
If that doesn't work, if the wafer is degrading around the stoma because of the stool, you may want to use paste or Eakin Seals to protect the area. You may want to use one of these products now, at least until you get that area of skin healed. I like Eakin Seals, especially while you're red and sore, they seem to help heal the area, and the paste has alcohol in it and may burn a little. The Eakin Seals can be expensive, but you can get some samples from Convatec that may get you through the healing process then you can go to the paste.
The beginning is always hard until you figure out what works best for you. Good luck and keep us posted.