I'm a big fan of Positive Psychology approach to education, reinforcing positive behaviors instead of discouraging negative ones.
When Moses is scolding the nations for the sins [of the previous generation!] I haven't noticed much of positive feedback for all the good things they did, like agreeing to exit Egypt and fulfilling all the Pesach obligations, crossing the Red sea, singing the Song and believing in Moses and G-d, for their exceptional behavior at Mt. Sinai, donating for the Mishkan, building it and bringing sacrifices, 40 years of studying Torah and fulfilling Mitzvos while being convicted to die at any day. For fighting the wars they were commanded to and they even killing each other when commanded to.
And all Moses chooses to discuss are the moments of their weaknesses.
Why is this gloomy approach?
NB, I know that your first move will be to search feverishly for counterexamples, but I'd like to see the big picture.