Don't feel bad, sometimes these thing happen...
When a fish gets stressed (sick for 2 weeks) it can lower the immune response setting them up to catch anything in the tank, columnaris is in the tank all the time waiting to strike, the water change can decreased the number of bacteria, so water change is the least stressful compared to high bacteria,
It often will show itself on the face and mouth first, you want to keep up with the daily 100% water changes, add an airstone, lower the water temp slowly to 76F and add 1tsp/gal of aquarium salt up to 3tsp/gal.
There are two strains of this gram neg bacteria, a fast acting type that can kill the fish within 24h and a slower moving type that the fish can recover from, it like warm water, low to no water movement, high salinity, dirty water, so you need to make it hard for the bacteria to thrive by lowering temp, add salt and airstone and clean water. Using a broad spectrum antibiotic can be helpful and it needs to be give by mouth or injection and usually the fish will stop eating, antibiotic in the water can help but usually don't once it has staged out.