What do you think is the main reason of your BLUNDERS ?

Is it:

A. Time Pressure

B. Overconfidence

C. Calculation Problem

D. Board Awareness

or something else?

Most times it's a combination of A and D

B

Most of the time, for me it's clearly B. Getting a winning position is often easy but getting the brain to avoid laziness (which reduces levels of A,C,D ) is a challenge

I will say though, that all the other points are valid, too - these are more the blunders which will be fixed simply by improving your chess skill via studying and playing

Just moving too fast.

For me it’s usually just moving too fast. Sometimes I make a move without really checking what my opponent can do in response. When I force myself to quickly look for checks, captures, and threats before every move, the number of blunders drops a lot. 
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75% C 15% D 10% B

c and d, sometimes b

Fatigue

only two things ever caused me to blunder in a while: 1. time pressure 2. being rushed to do something making me need to finish my chess game faster.

A. Time Pressure, mostly in bullet

C, D & I'd add high blood sugar too, it seems to sometimes create temporary brain fog.

40% C & 60% D

Being distracted, LOL.