more thinking and depression

I've been there too buddy. Fantasies and thinking about porn are somewhat a smaller version of watching porn. They give you the dopamine rush your brain needs to feed the urges, and you probably end up relapsing...
Yes just like you said, thinking about trying to stop thinking about P is actually thinking about P. So your best bet is to stay away from all that and to keep yourself occupied with something else. This is exactly what I'm doing now, I'm not even thinking that I'm on a Nofap challenge! After a while you will be able to become the master of your thoughts and redirecting every sexual thought to another healthy thought.
And yes thinking about sex without any visual images is fantasy and will surely generate an urge to fap. You need to cut all that out.
Best of luck brother.

 

make yourself busy in some useful stuff which needs some brains to complete. challenge your brain with the problems slightly beyond your reach (or atleast what you think of beyond your reach). let stretch a bit. sooner or later you will find your fantasies washed away. good luck brother.

 

Just like @ffk said. And it's a matter of time before sexual thoughts become weaker.