You didn't fail him. You gave him a year of life that he wouldn't have had otherwise. You gave him 12 months of care, kept him in clean, warm water and fed him good food.
What about doing a community "fish care workshop" for a local group? It could be any type of group in your local area. (Book club, social group, school group, church group, etc.) You could bring a few Bettas (or other fish), and explain how to care for them properly. That way, people who buy fish would be more likely to get the appropriate tanks, heaters, foods, which in turn would make their fish happier and healthier, and able to live longer lives.