ohh....dont give up just yet.

I think u need a good start, then its high flying all the way cos bettas are the hardiest fish i have ever reared. i used to bring RO water from my ofc.....cos i didnt want to use the tap water from my house. after awhile i got tired of lugging water home everyday, so i started with my tap water. left the tap water in a bucket for a week before i actually use it for my fish. they will be fine after that.

bettas can even go for days without food
i dont recommend that though hahahha

before you get new fish, you might want to empty your tank, and start over with fresh water, hopefully no more disease will spread.

post a pic of your betta, so that someone can give proper diagnosis for him