Progress?

I'm still working on that difficult flute passage--I spent 15 minutes today just playing the offending phrase over and over again.  I told myself I was just going to focus on playing at whatever speed I was able to do it correctly.  But yesterday I couldn't resist checking the metronome to see how close I was getting to half note=80.  I went from "just curious" to seeing if I could get closer to 80.  at 72, the pace was a bit slower than I was playing, but as soon as I went to 76, my fingers couldn't keep up.  I've told myself I wouldn't try to play with the piano accompaniment (companion CD) until Thursday (the day before my next lesson).

I don't seem to progress from day to day.  Each day I start practicing the piece, it's like I'm starting from scratch with that passage.  I have to slow down to play it correctly.  So I play the piece 3 times at a tempo that's slower than I need to play the rest of the piece, but that enables me to get through the difficult passage.  Then I practice the difficult passage over and over again and maybe get to the speed I reached the day before (using the same approach).  And some days my fingers feel stiffer than others, which makes the progress more uneven.  Today was one of those days.  So I sort of got back to 76 by the end of the practice.  I got to the point where I was playing the passage worse, so I stopped.  I seem to reach a point of diminishing returns with my music practice.  What's the line between being a quitter and overdoing it with practice?  Do skilled musicians have this same problem, or are they able to train themselves to keep going until they reach their goal?  Or is it talent, or some other mysterious element that goes beyond physical (and mental) effort?