HI and welcome to the forum
Have you tested the aquarium water for ammonia, nitrite, nitrate and pH?
If yes, what are the results in numbers?
How often do you do water changes and how much water do you change?
Do you gravel clean the substrate when you do a water change?
Do you dechlorinate the new water before it is added to the tank?
Can you post a picture and short video of the fish?
Set your camera's resolution to its lowest setting and tank some pics. Post the video on YouTube and copy & paste the link here.
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Male Bettas only live for a couple of years. Some might live to 5 or 6 but most die after 2-3. Yours might be old.
Try doing a 75% water change and gravel clean the substrate each day for a week. Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
If the filter is established and has been running for more than 2 months, then clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is not established, then do not clean it until it is established. Wash filter materials in a bucket of aquarium water.
The following link has some information about what to do if your fish get sick.
http://www.fishforums.net/threads/what-to-do-if-your-fish-gets-sick.450268/