How do I get my girlfriend to knock off this annoying habit?

I'm 30 years old, and my girlfriend, Mimi, is 23. We've been together for about five years, and most of the time, everything is great between us. Mimi has a bubbly, optimistic personality and is always there to lift my mood up on my worse days, she's attractive and affectionate, and she's a talented artist. She's been doing well with her career as a graphic designer too. All in all, Mimi's really something special, and I feel really lucky to have her.<br><br>The problem is that Mimi has this habit of talking to herself when she thinks she's alone. She'll just have entire conversations out loud using these high-pitched, squeaky voices. It doesn't matter if she's in the bathroom, changing her clothes, or even if she's just in bed with earplugs - once she thinks she's alone, the chatty voices always start. I know it's a little embarrassing for her too because she's always quiet when she realizes that I can hear, and she's refused to talk about it with me more than once.<br><br>I'm not really sure what to do. I've never brought it up before because I don't want to embarrass her. I'm also worried that she might think that I see her as weird or defective because of it, but the fact that she won't talk to me about it is annoying. It's a little embarrassing for me too because I've had a few embarrassing confrontations with roommates in the past about it.<br><br>We've been living together for over a year now, and it's gotten to the point where I'm finding myself getting more and more annoyed whenever I catch her doing it, especially when it's something obvious or loud, like when she's singing in the shower. I feel like she has no reason to keep doing it because she knows that I know about it, but she still won't stop.<br><br>How can I address this issue with Mimi without making her feel bad or embarrassed? I'm sick of her denying this stupid habit.