sounds like finrot - go to your lfs as soon as poss & ask for a finrot & fungus medication & start straight away.we used interpets fungus & finrot med (uk stuff) & worked a treat. i think theres even better stuff in the usa as you have free-er acess to over the counter antibiotics than we do here .
but remember to increase your tank oxygenation whist using meds( turn up your air pump) & follow the directions exactly dont overdose as you may cause more harm as they can effect oxygen levels & be careful if you have other fish such as loaches in there as they can be v sensitive to meds & you may have to half the dose. have you tried posting in the bettas section - loads of folk there with years of experience of keeping bettas & curing any diseases.
good luck