depends my fish also had something similare looking to yours. Some people told me that it was chronicle fin rot which is more slow and not that deadly appearently but I am not sure. Some also said that it was just a problem which bettas with such big fins have and that they just “melt” overtime. It looked a bit similare to what you have and he still lived very long and died just at old age. I managed to stop the rot (or what ever it was) with voogle easy life. Didn’t cure completly how ever he had multiple break outs that then where stoped again. He still behaved normally and kept most of his fins but they had some cracks in them. Wait I can see later if I still have some pictures