Betta Fish: Rip to my little guy

i tried to treat his fin rot and dropsy and he couldn’t pull through. he put up such a fight gonna miss him. i feel like im doing something wrong. my other boy bobby passed away about 2 months ago, his tank cracked and spilled all over my floor, i had none of his tank water to put him in so i had to use fresh tank water. he wasn’t able to adjust and passed. my dad trying to make me feel better bought me a new betta in hopes of making me feel better. (i had no cycled tank) so i panicked. we were going on a trip for the weekend the same day i got him. so i had to start an in tank cycle quickly and hope he made it through the weekend. he did! but i did come home to a ripped fin. that’s when he went downhill. he got fin rot, a few days later dropsy came as well. i got him on some meds but he wasn’t able to pull through. he was suffering and i felt so hopeless there was nothing more i could do. is there something i’m doing wrong with adding new fish? all my other betta tanks have been cycling for over a year and they are fine. but whenever i change water completely it never ends well. this might just be because of both circumstances i’ve had, but if i ever do get another betta i don’t want it to also have the same fate.