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.