I have a very quiet, mousy, childlike voice. I want a strong, determined, and assertive voice?

How do I do that? I am an adult (18) and I still sound like a child. I was bullied a lot as a kid and I was very withdrawn. Now that I am going out into the workforce, people have fired me because I just sounded and acted like I wasn't determined, like I was unsure of myself. In their words, 'like with every sentence (I) was asking for permission to exist''. It's not that I have low self-confidence, it's just that I'm not very social, and my voice I guess is just leftovers from my childhood. Links/info on getting a nicer voice and acting more determined in general, thank u