The shame and guilt from your past participation in hurtful behaviors does not excuse you from standing up now against those same behaviors.
Life is about growth. If you learned that your behavior was unacceptable, changed and grew from it, it is now your responsibility to use your growth in a positive and proactive way. If you allow yourself to be silenced because you are afraid that others may not see that your change is genuine, you are passively condoning the inappropriate behavior of others, preventing large scale change to happen.
Own your mistakes, but also own your change, and help be the change in others.