When practicing pieces of music on either the guitar or piano, I have almost always noticed the following pattern: to start with, each practicing session is better than the previous. However, there comes a time, possibly when I am limited from further improvement by lack of techique, that my playing starts regressing. My performance of the pieces I have been studying becomes sloppier as if I have lost what I had previously conquered (in terms of practice).

Have you come accross that pattern on yourselves, your peers, your students? Or is it just my practicing routine? Are there any suggestions on how I could prevent this regression and maintain an improvement, such that each time I play a piece, I at least play it as well as during my previous practicing session?

Background info: I am an amateur musician, without any formal (i.e., in a music conservatory) music training.