It's gambling, not trading or investing, so your confidence means nothing in gambling.
It's game of probability and luckiness, and whether you are very confident, it does not increase your probability to win with your bets.
With gambling, confidence is not important, the important factor is gambling responsibility. You must afford to lose your gambling capital, but even so you must gamble very responsibly to minimize your potential loss.
In trading it's the same, self-confidence doesn't mean anything there, how do you go against the current that builds? no, it's also luck when you get profit from it. Maybe what @OP means here is confidence in the slip we want to place, when there's a match between a strong & weak team, we definitely have more confidence to win if we choose a strong team because the odds offered are also small, this is related to the probability you mean.
Is actually the same thing with trading, because there's no self confidence in both of them. Just like as you mentioned I think what the OP mean in the confidence is when he bet a game that he is sure about outcome of it, because normally as a gambler when you bet on a team that you trust so much that is how you would going to have more confidence in it, and you might likely stake high in the game due to how confidence you have in them. but once you bet a team that you don't have trust on, that is how your confidence would reduce in the game while because you are not sure about outcome.