Lithium batteries typically have a lifetime of around 5 years. It can b extended a year or more, with proper charging techniques, and minimizing exposure to heat. Without more data from your Apple Watch, my best guess is the battery is at or very near End of Life (EOL) and needs to be replaced.
My wife has a Series 5 stainless steel and it’s about the same. The cost to replace is just $79 USD, but prices may differ in other countries.
Please contact Apple Support for more information about replacing your battery. If it more convenient you can make an appointment at the Genius Bar at a nearby Apple Retail Store.
Contact Apple for support and service - Apple Support
In the US please call 1-800-MY-APPLE
Contact Apple for support and service - Apple Support
Genius Bar Reservation and Apple Support Options - Apple