Can Anyone Help Me Diagnose my Betta Fish?

It looks like fin rot. It may be too late to fix it. That looks pretty bad. The only other thing I can think of is that it could be an ammonia burn. Are you testing for ammonia? Are you using carbon in your filtration? Using aquarium salt can help, but be sure that you are not using too much. Too much salt can cause abrasions on the fish too. Too much salt can cause immune defficiencies. I am not a big beleiver in aquarium salt, and I never use it in my fresh water tank.