Cali is right...the main ingredient of MLC is depression. It will take time for it to begin to show, but they all have it. Depression isn't always about feeling sad and not getting off the couch because people react differently to it.
As for wanting to live the single life...well...been there and done that one. She thinks she missed out on something and, of course, you are to blame for her not experiencing life and seeing just how amazing her life has been w/you.
Something or someone stunted her emotional growth a long time ago when she was a child. The word abandon is an important word here...she must have felt abandoned as a child. Which parent was distant or left her?
As for living alone...that won't last long. It's what she thinks you want to hear right now...but there is possibly someone waiting in the wings, either in person or on line that she wants to meet up with.
For now, try not mind reading...it will get you no where fast and just creates more doubts in your mind. Do not assume anything because assumptions don't always pan out.
Let's put the focus back on you and what you can do to enrich your life while she's playing the repeat teenager stuff. Okay?