Without knowing more about what you already doing, its hard to address your question w/ something on-point. You've gotten lots of good suggestions though.
You're flagging motivations could be a sign that you are overtraining in some way or getting inadequate sleep, calories, nutrition, SAD if you are in the far North or too much stress in your life. Over the years, I've found myself in the same place at this time of year & sleep & stress (demanding full time job) was my problem - you need more sleep & less stress during these big training weeks. If you can't make that happen, then I'd recommend you cut back on training a a week or 2 to recover. Unless you are pretty young, fitness is really about recovery.
If you can be certain none of the above are an issue, then training variety should be reviewed. Finding what will work for you will be personal. During the 4 week period we are now in the middle of, my training plan is 45% LSD, 20% intervals, 5% speed, 5% race/pace, 10% strength & 15% endurance (HR Z2-3). I mix RS & biking but mostly RS this time of year. But this plan provides lots of variety.
One thing I've added to my training plan is at least once a month do two 10 min. time-trials on a fixed course. Got this from Carmichael's Ultimate Ride bike training book. I do the TT on RS & use the faster time to track fitness improvements. This helps me be sure of my improvements & that what I'm doing works. It provides good reinforcement that the hard work is paying off.
At least quarterly, I do either a 30 or 60 min. TT on the bike or RS to check my ATHR ( the average HR during the last 20 or 45 min. of the TT). Again, more reinforcement.