Derlend: If the 'believe' isn't backed by the requisite chess skills, then 'no' the believe, in and of itself, will not be decisive, in fact it may be a detriment, if you think that 'belief' will get you results that more chess skills would deliver, whether you 'believed' or not.
Confidence in your game is important to maximize your skills, whatever those are, but is no substitute for having the requisite chess skills also.
BTW: typo, it's 'psychologic'.
In some games things are happening all over the board , both players has strenghts and weaknesses.
I lost today, beacause I believed that his strenght was so bad that I chose to trade down, to minimize.
If I had believed in my own strenght, I could have pushed it all in, and won.
To believe is a keyword. if you believe you dominate, then you dominate, if you dont believe it, you willl fade away.
Tomorrow is a new game. My plan is to belive that I dominate, and do it, because I believe it.
(there were other flaws with the game, but mistakes happened on both sides)
What do you think? Is what you believe decisive? Does it make the difference?