What are you practicing for? Why are you spending so much time and effort learning about the piece and the myriad ways to interpret a meaning behind the piece? What is it all for?
That was a bit more dramatic than I originally intended. But your question is hard to answer, because it lacks context. Do you have an upcoming performance and you find that you're burning out? Or did you find a cool piece that you want to save for a future performance but can't motivate yourself to finish? Or are you imagining music at a level that you simply cannot replicate on your instrument?
Let me instead share my approach to my music. I'm goal-driven, and I have two goals: to discover music, and to share music. I play a lot of music, mostly for myself. I love getting into the composer's mind (right now, I'm working almost exclusively on Tchaikovsky for some reason). I love finding meaning behind the work. And I love trying to match up the music I produce on the piano with the music I hear in my head. I never actually succeed, because my technical proficiency is not up to snuff. But like you, I eventually get bored with a given piece after a few weeks. This usually coincides with a lack of improvement in the piece. I'm not gaining any more insight, and I'm not playing the piece any better than I was before I started practicing, and I don't have a teacher to give me a kick in the pants. So I move on. When it comes to honing my skill on the piano, it doesn't really matter which specific piece I'm working on, as long as I'm strengthening the conduit between my mind and my fingers and my piano.
When it comes to preparing for a performance, I typically don't have an issue with motivation. I focus on a message I want to convey for a piece, and I work on honing my performance to deliver that acoustic message as clearly as possible. It feels kinda like improving a powerpoint presentation. It's often not fun work, but it absolutely must be done, and the payoff when everything goes well is fantastic. I don't know if by your standard, my music is "on ice" or "off ice"; when I have a choice in what I prepare (which is very rare), it really doesn't take me very long to re-learn the music-- maybe a few days. The rest of the time is spent working on my presentation.
But that's really because I don't frequently forget a piece. Every so often, I'll sit down and at a whim, start playing music that I haven't practiced for years. If I find one of my old anthologies, I sit at the piano for hours reminiscing. Some of the music is so easy that I can sight-read it now. Some of it I can stumble through. Others I barely limp through. And sometimes, I'll decide that maybe it's time to rediscover one of those old pieces and see what else I can learn from them.
I think this is a good point to stop rambling. I don't know if this answers your question, but hopefully it's useful. I think my take-home message is that for me, it became really easy to decide how to spend my practice time when I focused on what I want from my music.