Avoiding inner narration while practising Anapanasati
Inner talking is fairly standard with all practitioners. In your case it takes a form of instructions, in other cases it can be songs, arguments with imaginary adversaries, all sort of replayed conversations that had some emotional charges in them, or simply commenting on what one's currently doing.

All teachers I worked with always said I must learn to stop that talking.

At the end of the day, it takes a level of integration and control over one's mental faculties when you can directly feel where this talking is coming from and can let go of it, in the same kind of way as you relax a muscle.

Until you reach that level of insight and control, you can try several tricks:


reading meaningless mantras, out loud or in your head
holding your tongue and lips in a fixed position, e.g. tongue up against the roof of your mouth, lips closed
switching and holding your attention onto external auditory environment (background noises)
rolling your eyes and/or engaging your peripheral vision
humming 


At some point you will reach a level of integration when the talking will be very easy to stop. Until then just try to see how inner talking is (usually) connected with some sort of inner neurosis and see if you can relax that. Meanwhile you can try using the tricks above to stop it when it gets too annoying.