I was stupid enough to believe it.
This could actually be PARA or POR.
The preposition “para” is used in this sentence to express the purpose or goal of the action. In other words, the speaker is saying that they were foolish enough to have the purpose or goal of believing the statement.
The preposition “por” could also be used in this sentence, but it would have a different meaning. “Por” would express the reason or cause for the action. In other words, the speaker could be saying that they were foolish enough to believe the statement because they were gullible or naive.