I went through the exact same thing (waning interest due to not getting better) through 2015 after a decent 2014. I spiked the drop-off by buying a road bike, and it did the exact same thing you describe - interest up for a bit, and then the same sinking dread of churning miles just to get better at churning miles.

I picked up a subscription to Veloviewer, though, and that gave and gives me something to pick at. There's any number of stats on offer and achievement stars to tickle my gaming itch. Most recently I'm trying to widen my explorer score stats, I've got a couple of unexplored squares that'll push out my area of predation once cleared.
Highly recommended for adding more numbers to your cycling.