First the question of whether you are this voice must be resolved completely. Are you trying to shut yourself up or are there merely thoughts occurring to which you find yourself interested in.
Secondly, it must be clearly noticed that certain thoughts ARE appearing frequently because of the attention you give them. The struggle you have with them and the belief you put in them to be true.
These thoughts or this inner voice is a movement that flows just as emotions are flowing and merging into each other. Something is aware of this voice, these thoughts and these emotions though. Something which is not moving and is not struggling and is watching even the struggle to eliminate the voice itself.
The voice itself slows and quiets down as the struggle to silence it slows down. The further you begin to clearly see and know in your heart of hearts that you are the quiet stillness, you are the awareness upon which the voice occurs within; the less and less interest in the voice there will be. The illusion of there being a you to which the voice must be silenced will even fall away in the seeing that the whole drama was just mind struggling with mind.
The inner voice refers to the mind and the mind refers to the process of thinking. Any and all struggle to silence the mind only fuels it further. The nature of the mind is one of struggle and illusion. Put your attention into the stillness of your being and the stillness will grow.