I agree that a diode in the alternator is blown.

Most likely just easier and cost effective to change the alternator.
For now, disconnect the battery when the car is not in use to prevent the light from being on and subsequent voltage drain on the battery.

And don't be too hard on yourself. Most people have made that mistake at one time or another.
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"Differences of opinions should be tolerated, but not if they're too different' - Sharon Craig