"I used to think it were my narrow attention span, but later I came to realise it was just the teacher".
In this sentence, I first describe a reality that is contrary to reality. Going by the book, the word "were" in the sentence should describe a subjunctive mood. Yet, to my ears at least, it seems a bit off-putting.
So the question is: was it correct to use "it were" in this sentence, or had it been more correct to use "it was" (if so, then why?)