How do I stop feeling I am dictated by this destructive voice in my mind which seems to control me?

By reminding yourself "I am not what my mind tells me I am". The moment you realise you are who you are, blocking the negative voices in your head will become easy. Remember, you are wonderful. Not the person in your head telling you that you aren't. :)

I learned long ago that affirmations are a good way to re-structure the way my mind operates. Whenever I hear negative talk in my mind, then one of the many affirmations I've learned by heart comes to my aide. I can repeat it out loud or quietly to myself until the negativity ceases. Another thing which helps me, is to tell someone what I'm thinking. Once it's out, it loses it's power.

That voice is not real, it's part of your mind, it's a scary part of your mind and that's all. If you start hearing that voice, listen to music-

We all have that "tiny voices" inside our minds. We cannot deny that. But for atleast, try to be distracted. think of something else.

You control you mind, never let your mind control you in any circumstance. Because that is went you will need help to get your mind back under control.

Try and think of positive thoughts and hang on to these thoughts. Also you could try filling a jar with positive thoughts.