If you think that through and decide that way, then your precommitting method didn't work. The idea is that you must somehow now prevent your future self from behaving rationally in that situation - if they do, they will perform exactly the thought process you describe. The method of doing so, whether making a public promise (and valuing your spoken word more than $100), hiring a hitman to kill you if you renege or just having the capability of reliably convincing yourself to do so (effectively valuing keeping faith with your self-promise more than $100) doesn't matter so long as it is effective. If merely deciding now is effective, then that is all that's needed.

If you do then decide to take the rational course in the losing coinflip case, it just means you were wrong by definition about your commitment being effective. Luckily in this one case, you found it out in the loss case rather than the win case. Had you won the coin flip, you would have found yourself with nothing though.