I am going to paraphrase WetWebMedia here, and say, "I need more data."
To get a handle on what exactly is wrong with your betta, you need to know what the water quality is like (pH, hardness, ammonia, nitrite, nitrate-real numbers here), what sort of tank you have (is it an actual tank, with a filter and a heater and a size over 5 gallons, or is it a bowl, which you really shouldn't be keeping a betta in unless you keep the room at tropical temperatures and change out all the water daily), what sort of food you're feeding it, and so on.
However, just from your description, it sounds like a fungal infection. These only occur when a fish's immune system has been severely suppressed by aggression or a poor environment. There are proprietary antifungal treatments, but if something is wrong with the betta's environment they won't do any good.
Bettas require a temperature of around 77 degrees Fahrenheit or 25 degrees Celsius to thrive. They also require zero ammonia and zero nitrite, as these are lethal. Nitrate should definitely be kept under 40 ppm, preferably under 20 ppm-as low as you can go. They're not especially picky about pH and hardness, but if your pH and hardness are extremely low or extremely high that could point to serious problems.
Bettas are tough fish, and can survive in poor conditions for some time, which is why people mistakenly believe they do well in cups or flowerpots. But over the long term, the poor environment will cause them to grow weaker, until they, as you probably are seeing, lose their brilliant colors and get diseases.