Mine was the same - a slow decline, likely caused by my unwillingness to stop pushing myself.
On the Bell scale, started around 60 maybe 25 years ago (triggered by virus or vaccine - I'm not sure which). By ten years ago it was down to 30 which was miserable enough. Then a bad extended allergic reaction dropped me down to 15, and been there for over five years. The constant headache / head pressure, cognitive crashes, etc - that's the worst.
I know exactly how you feel.