Feeling very discouraged

Yes, the stoma can stretch either naturally over time or with constant/consistent "pushing the limits" with food volume intake, you're correct. It is the stretching of the stoma that is often the cause of over-eating, regain, and possible revision as food then passes more quickly from the pouch into the intestine. The pouch also stretches naturally over time - it can barely hold a couple of ounces as a new pre-op but expands/stretches a little as time passes to accommodate a larger volume.