When it comes to the character of Laura, we could certainly expand on the theme of "loss of innocence". This is because Laura, a woman who is over-protected, has an extreme social phobia, and a very low self-esteem, depends entirely on her "younger daughter" status to justify her lack of self assurance.
She certainly does not do this on purpose: the dynamics at the Wingfield home are dysfunctional and include role reversals, and the stepping on boundaries. Laura's role has remained that of the protected little girl and she seems almost encouraged to adapt to it in order to "protect herself" from being ridiculed about her foot.
When Jim visits the home, Amanda Wingfield makes it all appear (in her mind) that the visit is to be a date where Jim is actually a "gentleman caller" of Laura's. Perhaps Laura, in typical fashion of co-dependent people, allowed herself to believe a little bit the fantasy of her mother.
As Jim and Laura are alone and she shows him the glass menagerie, she experiences for the first time the feeling of being an adult woman that could actually be in love. Jim made her feel unique and special, which detached her completely from her role as the "poor little sister".
When they dance, Jim uses key phrases that really reach Laura's psyche
JIM: Let yourself go, now, Laura, just let yourself go.
JIM: You make me feel sort of - I don't know how to put it ! I'm usually pretty good at expressing things, but This is something that I don't know how to say !
JIM: Has anyone ever told you that you were pretty?
All of these are obvious ways from Jim to have Laura fall for him. In his mind, he has never stopped being the High School hero that he no longer is. The biggest addition to his ego would be to have this girl from back in his heyday to, years later, still admire and lionize him. This is when the loss of innocence occurs. Right around this time, Jim has accidentally broken the unicorn of the glass menagerie. Laura thinks that this may be a "blessing in disguise", and that this may make the horse look less "freakish".
Like the unicorn, Laura has lost with Jim the what made her awkward: she is openly communicating with him, she is clearly in love with him, and she feels like a complete woman. Then, came the kiss, which sealed her transformation. All was going well until, moments later, Jim discloses his engagement, leaving Laura, literally, deflated.
The innocence of her thoughts, her high school memories of Jim, and of herself as a protected "child" have all gone away in a second and she is left with the knowledge that nothing will ever change in her life again.