'Only I knew that she could disappear...'

Interesting. You're not the first person to ask that question on this board, but I always hear that line as "The thought that she could disappear" which makes more sense. I think I checked the subtitle last time this came up and that confirmed it, but I'm not 100% about that. My memory of it though is that that is a mishearing of the line, but apparently a common one.

Reality is the new fiction they say, truth is truer these days, truth is man-made

I think it means she's scared Kitty may die. I mean, Laura was in love with her...

I don't think Laura is truly talking about Kitty anymore; Laura is talking about Laura. She is so deeply depressed, that she is terrified.

Her husband who loves her very much is somewhat aware of her illness, but doesn't realize the extent of it. I interpret the scene as having two conversations going on: Laura's own illness eating her alive (a conversation we don't hear out loud), and the very surface-level conversation about Kitty with her husband. It is almost painful to watch Laura force conversation and try to appear "okay." I imagine this is just one small glimpse of how every day is for her character: the chaos going on inside, and going through the motions on the outside.

That's just my interpretation though. I have depression as well, so I could just be projecting my own feelings into the movie. Interesting topic!