Hi there ! I just went through the same thing about halfway through acoustic 1 in my most recent lesson. The piece was quite challenging and I must have played it at least 500 times. It did improve albeit very slowly. I was able to play along pretty well at 75% speed and occaisionally at 90 % but full speed was out of reach.

Usually I like to be able to play the song tutorial at full speed but in this case , felt that trying to master the piece was just going to become counter productive at some point and decided to move on. I felt that I had a solid grasp of the concepts in that lesson and able to play the piece well enough to allow myself to proceed to the next lesson. I figured that I could always circle back to this and other lessons in the future if I desire with the knowledge that playing this piece 500 or so times would help me to become a better player down the road.

Hope this was helpful in some way.