I have never had a nervous breakdown but I think I came very close once.
It happened because of an inward fight, a mental fight which was not so much a verbal fight with myself as an emotional one. I was torn between two choices and my heart told me I had to take one road and my 'sensible' mind another. I had to take a risk, a major step in life and I was afraid to take it.
The tension came as I vacillated and procrastinated over a period of some months. I was in a very responsible job at the time which requied me to concentrate and I found that I could not. Inwardly I was rebelling. There was a sort of inner rage that I could barely contain. In a nutshell I was torn between the security, prestige, excellent money and the like and an inner prompting which today I know came from own spiritual essence. Move!
Evenutally I did. I put in my resignation from a career I'd once loved but which had now soured. When I finally posted that letter of resignation and knew there would be no turning back it was if a great weight had been lifted from me. I was free. There was risk ahead but I was up to it.
I changed jobs, changed countries and started a new life with my wife and three young children in a new land. But in retrospect, I feel had I tried to hold on without making a decision for any longer I would have fallen ill - or maybe had that nervous breakdown.