I'm sorry for your loss. I've only had three bettas and have had luck with all of them. I've never really had to go through having a betta die unexpectedly, though. I still have the second two (one I've had for a year and the other I've only had for about a month.)
The one I had in elementary school actually died while I was at camp, but he didn't die of any sickness. He was in my parents care at the time. My cat killed him and my parents obviously didn't even check on him like I asked them to. I was at camp for five days and I know I didn't ever do water changes and I had him for a year. While I was at camp my cat knocked the lid off of his little one gallon tank and he disappeared. I had that betta, named Flipper, for a year.
We actually found him months later. His tank was on a bookshelf and the poor guy was dried up stuck to the back of it :( It must have been a horrible way to die. I had thought that my cat ate him but apparently she didn't. He either jumped out or the cat fished him out. I guess I will never know what really happened. I had also gotten him from a friend of my moms and who knows how long she had him. I don't believe she really kept up on water changes, either. The only time I recall changing the water was when my cousin dumped an entire thing of colored sand in the tank from a little decorative thingy that I probably made the year before at camp.