I wrote a tiny post on good, bad and poor decisions (blunder).
Making a bad decision is understandable. But why do we make blunders. When we are in the capacity of making a good decisions, why are we converting them to poor decisions? Do we like shooting at our own foot? Of course, not.

Our brain has two minds. The truth is that one mind is rational and the another mind is emotional. Emotional brain is mighty and powerhouse. The Rational (animals don't have this) one is tiny and succumb when there is a tug of war with emotional. First we need to understand that emotional mind is capable of making "poor" decisions as the decisions are taken based on "what it likes" but not on "merit". But the rational part takes decisions as it names says "rationally".
Understanding that we have two minds and accepting that we take emotional decisions and constantly using "rational" side would help us make better decisions. Making better decisions is a habit. Unless anything forms as a habit, the results ought be inconsistent and unpredictable.
Bottom line: Practice to use rational side to make "right" decisions. It is a long term process.