I teach English to foreign students. I also speak English and French. In French there is no "Present Perfect " tense so it is easy for a French person to make a mistake with its use. Actually J-M meant " I was in this state of mind for months....." Past Simple tense for a "state" which has now finished  .

The post was addressed to the whole forum but also to especially myself. Jean - Michel knows that I have recently  become a " lost sheep " ( sarcasm Darren ) and he was showing, I thought, some insightful understanding of my feelings. In all honesty I had to read that sentence twice and realise the mistake which I knew he had made . ( as we have been in touch by E Mail and I know that he isn't confused now !)

The " Past Perfect "is a narrative tense  used when there is a change in the time period within an idea or sentence which is not the case here.  

( eg. I had been in this state of mind for months previously but I wasn't any longer.......)

HERE ENDETH THE ENGLISH LESSON !!