Where we went wrong: the Shuttle
The original STS design by H.H. Koelle was completely reusable. But that configuration was scrapped because of DoD payload requirements.
Then the final shuttle configuration was supposed to have pressure fed liquid boosters instead of the SRBs. Standing alone, these would have provided extremely low cost launches, and no one could have matched their price per pound to orbit. When these were scrapped von Braun left NASA.