Final answer:
In the sentence 'I found it difficult to believe ___ intensive pronoun,' the blank should be filled with the intensive pronoun 'myself' to emphasize the subject.
Step-by-step explanation:
The blank in the sentence 'I found it difficult to believe myself ___ intensive pronoun' should be filled with an intensive pronoun. Intensive pronouns are used to emphasize the subject of the sentence and are not essential to the meaning of the sentence. Common intensive pronouns include myself, yourself, himself, herself, itself, ourselves, yourselves, and themselves. In this case, the pronoun 'myself' refers back to the subject 'I' and emphasizes who found it difficult to believe.