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No explanation worked for me. I did a truth table, and said, "...huh."

You don't need to model all three door choices. Just say you pick Door A, not Door 1. The first door you didn't pick is B, and C is the other door. Then map the prize behind each of 3 doors, and whether you stay or change your answer.

Then count the number of successes for change versus stay. Spoilers: It's 3/6 vs 2/6.

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My experience was I coded a simulation in Python and got 50/50.

... this revealed a major flaw in my intuition that helped me to grasp it. I had subconsciously been assuming Monty sometimes opens the door revealing the prize instead of always opening a goat door. When I coded that into the simulation, of course it broke (because the player was now playing foolishly, looking at the prize and intentionally choosing the closed door that definitely still had a goat behind it).

This is an even better learning experience than coding it right the first time, and I am so happy you had it!

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