We share some common traits....

Ride mostly solo............check
Ride 15 - 40 miles..........check
Ride 4 days per week......check (on average)
Riding competitive..........check
Waning motivation.........check

For myself, it was important to set clear goals. Mine is to ride faster and lose weight. Except the rides started becoming a test of endurance and definitely not fun. It got bad enough, that a few rides were almost over after riding only 100 feet. The bike started symbolizing work instead of freedom and fun. So I started changing how I ride to bring the romance back.

Here's what I did...

Instead of hammering the pedals over the entire ride, I choose particular locations to work out; hammer a hill, short or long intervals on a flat, maybe high cadence spinning for X miles. Whatever workout I choose, I try to get it over early in the ride. This way, I can enjoy the remaining miles without any guilt or feel the need to challenge myself. At some point, I'll pick one day (or two) to ride hard for extended miles, but for now, the short or mid intervals are working.

I believe having a riding partner or group would also help keep the motivation up. But, I haven't found anyone close, so I'll continue enjoying solo country rides while watching farm life, the animals, changing weather and seasons after the workout is over. The bike is fun again.