Speak your truth whenever it's needed. Don't bottle up things. Your words might be the most valuable in the room. Don't hesitate to put forward your opinion. Be confident. If you don't feel confident then fake it. It actually works.
Be assertive of what you say.
I feel the same all the time, I want to react or say something but end up changing my mind and not saying it . And I don’t think it is something you can change overnight. I think you can form a close circle with your friends and family with whom you feel free. Spend more time with them and build up your confidence. And then gradually, come out of your comfort zone.