Rating the difficulty of a piece is not an exact science, don't get hung up on levels - it's just a guess. It's better to put more stock on how much of a challenge it is
for you. Everybody has different backgrounds, strengths and weaknesses anyways.
It sounds like this piece might be a bit out of your reach for now. If you've been working on it for weeks and can't get it to your satisfaction, you're probably getting a little burned out with this one. That's nothing to worry about (personally if I'm able to get a piece sounding 'just right' after a few weeks I feel it wasn't enough of a challenge

), it's all part of the learning process.
Of course I may be wrong, I'm reading between the lines here.
My advice would be to pick an aspect that you feel you're weak with and focus on that for a while. You mentioned rhythm and speed a few times, maybe that would be a good area to work on?