A lot of the time, bettas are sick from the shop. My first boy, Felix, died within 24 hours, and I did everything right.
I don't know how old Aphrodite was (about 3, I think, but she died after about 6 months). I had Athena for about a year and Apollo for much less. I've had my sorority 6 months and have lost two girls (one to unknown causes, possibly violence, one to the mysterious disease that claimed everybody else).
You'll find the most frustrating thing is when you do everything right and give your fish amazing lives and they only live a short life, whilst you'll have friends keeping them in unheated, dirty, tiny bowls that live for ages. Of course, it doesn't mean they are doing it right - they just got lucky.
Keep learning, keep trying. :)