Hopeso, I still cut my wafer after 15 years! I used the template that comes with my wafers and slipped it over my stoma and the precut are either too large or too small. You want as close a fit as possible...hopefully only 1/8 of an inch of space all around. The paste you put on the wafer will fill in that area nicely.
After surgery, the stoma is swollen and generally misshapen. Mine was oval for several months and steadily got smaller and rounder so I had to keep checking the "form" I made out of cardboard and changing it to make sure it was small enough to protect my skin. After several months my stoma stopped changing and it's stayed the same ever since. By the way, I have used the Convetec two piece flexible system all of these years and have really liked it.
The burning feeling that you mentioned usually means you need to change your appliance. (Actually, you need to change your appliance before you feel the burning!) The output contains a lot of enzymes that break down the food you eat and they can break down your skin too. Sounds like you need to "snug up" your wafer. You should be able to get four days wear out of it.
I know how expensive the appliances can be but, if you have medical insurance, start hounding them about paying for the appliances. I did and got Aetna to pay for my appliances every month! (I still had my copay.) I kept calling them and explaining my surgery and asked them if they thought the appliance was necessary! I told them they covered artificial limbs, pacemakers, etc. and that ostomy supplies were absolutely necessary for ostomates. I explained how prejudicial it was that ostomates were excluded from help. We need appliances and can't live without them. I never gave up. After several months of hounding and nicely explaining the plight of an ostomate, they relented and covered me as long as I had their insurance!
Best of luck and, if you need more help, just ask and all of us will do our best to get you over this hump.
Sherrine