I'm sorry for your loss.
They are part of our families, we all know how easy it is to get attached to our fish!
Don't feel guilty. Some Bettas are tough and can survive anything, and others get sick very easily. When they go that suddenly it's hard to figure out what happened, but you probably you did absolutely nothing wrong at all.
My BettasDavros, multi-colored veiltail maleSpock, blue veiltail maleOswin, red veiltail femaleCastiel, blue and white Delta maleGandalf, marbled veiltail maleDr.Horrible, yellow veiltail male