tomUK said:... Is there a cure?!
Sure, mix things up, don't go hard every ride, even mix up the terrain you use for harder days and above all listen to your body and don't be a slave to your schedule if you're losing enthusiasm.
For instance limit the number of hard days per week to two or three which includes any focused timed interval work in L4 or above and the group ride which as rbarker points out may be digging a pretty deep hole that's leaving you too fatigued for other quality training. So maybe you do the group ride or maybe you skip it for a few weeks but either way only a few days where you really work yourself with focused goals (staying with the group, holding 90% of FTP for several 20 minute efforts, doing a set of 5x5 L5 intervals at 110% of FTP, etc.) and on the other days do a couple of hours of Tempo which by definition should be your own best 'fun and fast' pace not someone else's. Most folks really enjoy a couple of hours with a lot of it at 75-85% of their FTP but it get's a lot tougher to do the focused and timed L4 intervals or chase faster riders at their best pace. And perhaps some days you just roll around for a few hours at mid to high L2 pace and just enjoy covering some ground on your bike without the need to focus on holding any particular pace.
And of course take a day or two completely off the bike or with the discipline to spin really easily for a very short ride expecting no direct training value but just to expedite recovery and spin out the legs. If you can't help but pushing harder on those rides, chasing down other riders or jamming up hills then stay completely off the bike on those days or you'll start running the risk of lower quality work on the important days and the sort of mental burnout you're worried about.
The point is that a schedule with a few focused work days, a few mid level efforts at your own best Tempo pace and some easier and complete rest days isn't hard for most folks to sustain after they've ridden for a while. We can still get a bit fried when life and work stresses creep into the picture but no one is paying us to ride bikes so listen to your body and your moods and be willing to take an extra day off or drop an L4 session back to Tempo when you feel the need.
If you like riding a bike and want to ride it faster it's not too tough to make a schedule, allow some flexibility and improve without a big burnout risk.
Good luck,
-Dave