I have the EXACT same issue, I am very soft spoken and when I try to speak louder either, I don't, or it comes out as (or feels like) a yell or scream. I've tried a few things that have helped me, one was recording myself talking and then listening back. I dunno if you're like me and have an issue with the sound of your voice but getting used to it helped. Also practicing talking at a louder tone without anyone else around makes it more comfortable to do while around people. It's a mental thing as much, or more so than, a physical thing.
After I drink I gain a mini confidence boost and my voice tends to be a bit louder and pronounced, like a few weeks after. Basically what I would recommend is practicing using your voice at a more medium tone. If you're by yourself with a mic you can try all different levels (you know you're capable of different tones!) and find one that is better, then just keep talking at it until you get used to it. Listen back to yourself talking so you get used to it.