Originally, the game show host has two doors and you have one.
Therefore, the game show host has a 2 to 1 advantage over you.
So, you want to switch with the host and then you will have the 2 to 1 advantage. (The fact that the host shows you a losing door does not change the odds; it just confuses the problem.)
If you keep your original door, you have a 1/3 probability of winning.
If you switch, you have a 2/3 probability of winning.