Believing only one thing matters, when in reality, it was a chain reaction. What's the bias called? Assume I had a business. That business would've been very close to be sold to a bigger company, if only I didn't mess up some of my papers. Thing is though, I didn't really only mess these papers. The issues started years ago when I thought I'd be smart and not fully fill out all the forms and so on. In time, it all added up, but given that it was some precise papers at the end that got me, I'm prone to say "ah, if only this [most recent evil] didn't happen!".

What bias am I guilty of?
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, and reduction fallacy, is
  a fallacy of questionable cause that occurs when it is assumed that
  there is a single, simple cause of an outcome when in reality it may
  have been caused by a number of only jointly sufficient causes. It can be 
  logically reduced to: " X caused Y; therefore, X was the only cause of
  Y" (although A,B,C...etc. also contributed to Y.) (Wikipedia)


The terms bias and fallacy are used to describe how being unaware of deceptive logic or prejudice results in wrong conclusions. In the scenario in the question, one is consciously trying to interpret the situation in attempt to avoid the full truth, which can be described with the terms, such as:


Minimisation - a type of deception involving denial coupled with rationalisation in situations where complete denial is implausible (Wikipedia).
Belittling - regarding or portraying as less impressive or important than appearances indicate (Dictionary.com).