Luna, thanks for the message.
What to do about that finrot- Stop that treatment of Betta fix. It contains tree oil which can damage a Betta's labyrinth organ, and it often does more harm than good.
To treat it, the main thing, KEEP THE WATER CLEAN, which means more water changes. Also, put some aquarium salt in. Read the directions on the package.
I hope this helps and your fish gets better. :)