I have noticed this kind of effect on myself -- I start doing something... the improvement is very slow... I convince myself that the improvement is zero... so I stop doing it... so I never finish it... which I take as a proof that my improvement was zero.

Having unrealistic expectations makes it even worse, because the higher expectation the more disappointment with slow progress.

Probably the correct solution would be: just record your improvements. I imagine something like beeminder, just without the deadlines: create a graph for something, enter data points, see the visualization, celebrate when you reach some goal.