The fact that positively-reinforcing is causing a self-fulfilling prophecy, the second one seems like the observer-expectancy/Rosenthal/Pygmalion effect to me. There could be a more particular form of it but generally, I think it fits the scenario.
The name comes from the famous Rosenthal-Jacobson study:
Rosenthal, Robert; Jacobson, Lenore (1992). ...
This sort of experience is probably better termed a heuristic rather than a bias. The two can be related in some cases (as heuristics can cause bias when the underlying model does not fit the real situation).
From the Wikipedia page linked above, a heuristic is:
any approach to problem solving or self-discovery that employs a practical method that is ...
The described scenario is about ascribing the problem to a single mistake, while the real problem was long-term wrong behaviour. From the list of cognitive biases or fallacies, this is the closest what I could find:
Fallacy of the single cause:
The fallacy of the single cause, also known as complex cause, causal
oversimplification, causal reductionism, ...