Right you are, Uncle Dave, from my experience at least. Musical progress is not linear, or even incremental (sp?) It is occasional, unpredictable, and rather fickle. Like all good Zen masters, we should enter our exercises without any expectation of progress, and accept that we are moving forward whether we can detect it or not. Scatter your more challenging pieces with confidence-builders. Violin to me was especially unforgiving; I'd take one day off and move backwards one week! Just keep slugging it out. Even a poor practice session will make you a better musician.