Sorry for your loss. Don't get discourage though, we all went through that. You did everything you could to help her. And i saw your last post that you just made, you didn't do anything wrong. Bettas get sick just like a human being . And you always get a chance to buy sick betta in the store. So i think she was sick to begin with. Sometimes we can't help them but at least she was loved and in the clean water which is more that she would get in the store. I hope you will rescue another betta. Just make sure you disinfect the tank and everything in the tank. Let us know if you need any instructions on the disinfection.
And when you go to the store try to check a betta for any sign of the disease before you boy it. Let us know if you need help , we can give you a few tips on the symptoms to check for when you buy a betta.