C4F,
I tried to PM you, but I keep getting an error message so I'll post here.
You could try a short salt bath and see if that helps.
Take a gallon of tank water and place it in a bucket (used for fish only). Place a tablespoon of salt (non iodized table salt, kosher or epsoms will do) in the water and stir it well until it all dissolves.
Place the betta in it for up to a half hour monitoring it for signs of stress. If any stress is noted pull the betta out immediately.
If no stress, after the half hour place the betta back in the main tank and watch for any signs of improvement.
let us know how it goes.
CM