How do I find my own voice again after speaking for my boss for years?

You have spent so long sounding like someone else that the line between you and them has gone faint. That is not a failure. It is what happens when you do a hard job well. You learned to carry another person's mind so closely that yours went quiet in the background. Your own voice is not lost. It has been standing aside, waiting, while you gave the stage to someone else's words. The thread is still there. You just have not pulled on it in a while. That is a thing you can change, slowly, on purpose. Start small and private. Write a few sentences each day that are only yours, meant for no executive and no audience. Say what you think, not what would be approved. Do this without judging it. Over time the voice you thought you lost will start to answer back, and you will recognize it as your own.