Voice is all about breath, and breath is all about space in your torso, i.e. your posture.
The smallest thing you can try when speaking up is simply lift the very top of your head, like it's a balloon filled with helium.
In fact, see what it's like to practice that once or twice a day. Feel the top of your head with your fingers. It might be a little bump (with a tiny dip in the middle).
Then use your mind to lift it, and just notice what it feels like.
More space inside?
Then decide what you feel like saying from that position.
Dave Wolovsky, MS CAPP
Relationships Coach, answering the internet's questions about all kinds of relationships.