Bettas can be fickle fish; I kept one alive for four years once, most people I know only manage 2-6 months, and my most recent Betta kicked the bucket after only 15 months . . . he developed a "food aversion" which meant I had to feed him live brine shrimp instead of pellets. I don't feel very guilty announcing that I was relieved to find him belly up.
The more I research Bettas, the more I'm convinced that they really need larger tanks complete with water circulation and heaters - badasses may survive in the smaller bowls, but it's not an environment they're likely to thrive in.
Short version: you didn't fail at fish keeping, the general public is just very very poorly eduacated about Bettas, which keeps some Betta breeders very happy to be in business. Peh. :/