Maybe it's time to spend more time with those 'simpler' exercises, trying to squeeze a bit more out of them. Look carefully at how you are playing them, beyond just getting the right notes out in the right order. Use them to push the boundaries of your own musicianship and explore the musical possibilities. Look for ways to play them in different ways or with different articulations - loud/soft; legato/staccato, strict timing/rubato; or transoposing to different keys, or up one octave....
It's like once the boredom was established
...
This time, not doing those exercises, I was able to move through the more interesting exercises much more fluidly, and I feel like I got a lot more done even in less time than the last session.
I'm currently working on some stuff from the Popper School of Cello. This is the first few bars of Ex.19. as written in the book:
This is just the notes of the first 2 lines with no articulation or dynamics, such as you might see in the HLBM. I've put it into 12/8 to ease the transcription process and moved the low Eb up an octave - I will probably transpose to a more suitable key at some point:
Here I'm looking for a different articulation and phrasing at a slower tempo trying to create a more lyrical character. RIght now this is not it, but progress is being made, even if most of the ideas turn out to be bad for one reason or another. There might easily be several hours work just exploring the possibilities of just these first few lines, but I'm confident something of real value will emerge: