@mister
For me, I found that changing what I was doing adjusted my motivation. I was pretty much doing the same thing as you, plodding through Core, and my motivation just kept sliding. When I stopped doing that, took a break from learning anything new for a few days, and then started back by doing reading/watching anime/listening to podcasts/jpod, my motivation jumped back up. Doing the Core grind sucked my energy out, so that doing those other things felt like an added chore.
Now, I’m not adding as many new words to my deck as I was before, but I feel like I’m learning much more, because I feel good about learning. It might not work for you, but it doesn’t sound like what you’re doing is working for you right now either. Change can be good. :)