You just used a logical fallacy, known as denying the antecedent. I stated that you can only win if you are sure you won't, which in proper logical form is: IF (you are sure you will win) THEN (you will not win), which can be transformed via modus ponens (denying the consequent) into IF NOT (you will not win) THEN NOT (you are sure you can win). That last bit in regular English is: If you win, then were were not sure you would.
In an IF...THEN proposition, you cannot assume that, because the antecent (the IF bit) is false (denying the antecedent), then the consequent (the THEN bit) is also false. Think about this: IF it is sunny THEN my clothes will dry. Let's say it is not sunny, and you throw your damp clothes in the tumble dryer. The antecedent is false, but the consequent is true, yet the proposition itself is still true - had it been sunny, you would have put your clothes out and they would have dried.
In the same way, looking at the proposition IF (you are sure you will win) THEN (you will not win) and deny the antecedent (I am NOT sure I will win) then the consequent is not necessarily going to be false as a result.
That probably made no sense, but my critical thinking skills have been going rusty, and I needed an excuse to exercise them.
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