First,,, I love Billie Eilish...
Second
isn't it lovley all alone- Means that she would rather be alone because she couldn't get hurt if she was all alone... And that being alone is her safe place where she knows she is not gonna get hurt.
Heart made of glass- It means she has a fragile heart, that she is sensitive, and that she has probably been hurt before.
My mind of stone- (if you have ever heard of the phrase stone cold) I believe what she means, is that her mind is gone, that she can't be hurt anymore, like her mind is dead, she doesn't have to deal with the demons in her head.
Hope this helped!
lemme know if you need any more help with any of the lyrics<3
