Bettas are really tough, they're break your heart - I kinda resolved not to get a betta when I made friends with a betta breeder. He was great and loved his fish, gave them the best care possible, but also had a tendency to try to rescue struggling pet store bettas. The heartache I saw him go through with trying to save them was awful.
They seem to be a weaker fish in general - like guppies, the ones I used to keep. They broke my heart a lot too, because no matter what I did, the first trios and pairs of pet store guppies just kept dying. Perfect tank parameters, lots of live plants and doing everything I could to save them... it was only when I came here I learned that pet store guppies are no longer the robust, hardy, easy beginner fish they always had the reputation for being. Now being mass produced in fish farms abroad, often using sea water to save money, so the fish were raised in brackish water and had to adapt to freshwater when shipped here - frequently infested with various diseases and carrying heavy worm burdens... not being bred with health and longevity in mind. I was warned about pet store guppies, that if I got them, to worm them in quarantine, and not to expect too much, no matter how much I tried. That if you could get some fry from them before they died, at least the fry would be hardier.
Stuff I wish I had known before I lost many guppies, feeling like a fish murderer and that I shouldn't be in the hobby!
I suspect there are similar problems with bettas. Bred for looks rather than health, and those huge fins are beautiful, but easy for them to get damaged just by the male flaring -and then fin rot sets in. That betta breeder told me that even having a fish store betta survive for a year is doing well, and no one can say that you didn't do everything you could for him. This isn't your fault, okay?
If you decide to go for a betta again, definitely look for a hobbyist who breeds them at home, and is happy to talk to you about their conditions, the parents etc, share photos. Consider that the shorter fin bettas like plakets tend to have less problems with fin rot, and less issues from carrying those heavy fins around, and consider females too (although I'd suggest not a sorority). Some are just as stunning as the males!
I really, really want you to know that it isn't your fault, and that you went above and beyond. I also really wish there was more we could have done to help you, and your fish. But dropsy is dreaded because by the time you know it's there, it's almost always too late