Sorry I don't know what to advise because sounds like you are already doing wheat I would try. The time I cured a fish with tail rot, I did 25% water changes twice daily, kept the tank dark,, added aquarium salt and a medication- I used melafix (it wasn't a betta) but I think the frequent water changes did more than the meds. In my case it took a week for the fin to quit deteriorating, and then another week for it to grow back.

 

In my betta's case, water changes and Sera Baktopur helped in healing my betta's fin rot.

 

I lost two bettas to fin rot (actually no, to dropsy afterwards) I tried many treatments on both of them, nothing really got rid of it but to cut their fins. Afterwards it grew back, but I waited too long to cut and their organs were damaged, they slowly but surely contracted dropsy (changed the water everyday in hospital tank as well). One recovered from the dropsy, but couldn't pass food after that, and I had to euthanaze him.
So my advice: drop the meds, really, don't do more of it and cut his fins before the meds damage him too much. It will start to grow back after a couple of days if you keep him in clean, heated water. If you don't have the gut to do it without anesthetics (it can be stressful and damaging as they tend to want to jump and wiggle) you can search "betta grooming" videos on youtube, there's a tutorial on how to do it properly by an asian breeder (his fish doesn't have fin rot and I don't approve on cutting the fins of a healthy fish, but it's very well explained and showed).

 

The problem is that the fin rot isn't only located at the edges of the fins. As I Said, the fins are shredded and start tout split connecting to the body, so I wouldn't know how to Cut to remove all of the fin rot.

 

We have a betta that is a frequent tail biter, and dosing with stress coat really seems to speed the healing process along. I know fin rot is indicative of an infection as well, but doing your water changes with stress coat (if you aren't already) instead of something like Prime might be that extra bit that you need to kick this thing.

 

You could try doing a 100 percent water change, well technically save 50 percent of the old tank water into a bucket then get rid of the rest and then clean your gravel and ornaments with hot water to sterelize and to make sure any harmful bacteria are washed off. Then rescape and refill along with the old tank water so that your fish can recognize it, and puts less stress. You can persist with the salt treatments, just make sure its aquarium salt or non-iodized salt and make sure he/she is being treated in a quarantine tank or container using its tank water. Another solution that helped is using Indion Almond Leaves, you can get them on amazon for 10 bucks. They're miracle workers, it releases tannins into the water like tea which gives off anti bacterial properties and helps with the tanks water pH. Here are some before and after pictures of my betta fish, these were a span from May 5th - June 1st of this year and still growing.