Also, it's kind of hard to determine exactly how old many bettas are when they come into the pet stores. If I understand it right, bettas are not the longest-lived fish out there, just a few years is all I've ever gotten from one. If the fish you get is fully-grown when you take it home, you might be getting a two-year-old fish and not know it....I've always tried to get the smallest one I could find, without the "frilly, spiky" fins that are so pretty, because the big guys with the pointy fins are usually old and only live for a few months once you get home with them.
It's really sad, most people see goldfish and bettas as "throw-away" fish, and they're really some of the best pet fish out there.
If it were any other type of fish, I would suspect a compatibility issue, but unfortunately, my experience has been very similar to yours, and it drove me away from keeping bettas for a very long time. I've only just started keeping them again (after having some very good luck with gourami. I figure if one member of the family likes my tanks, maybe the others will, too... I got my first betta in years last month...)