Challenging and changing that harsh voice, is like changing a habit you've picked up and carried on for years.
It takes time, understanding and a whole heap of self-compassion.
It's no easy feat but practicing daily, even in little ways is conducive.
Some tips;
1. Give it a different name. Most inner critics aren't even us but originate from childhood years or society.
2. Make it sound funny/weird (for example give it Homer Simpson voice!)
3. Use the "what would I say to a friend" tool. It helps externalise.
4. Strengthen your compassionate voice, get it louder. Practice outside of times you feel triggered.
5. Mindfulness is a wonderful tool to help. @headspace have an exercise to help engage with self-compassion.
6. Be curious. Try to understand what tends to lie underneath the critical voice; fear of rejection, not being good enough, dismissal? So learn what your triggers may be to aid moving forward.