I don't think you are exactly her back up plan but in effect you are.
Her + you=
Much chemistry X Toxic relationship=
crazy about each other but 2 explosive
Her + her current bf=
get along +everything runs smooth=
a 'normal' relationship
she is weighing and leaning toward the normal and not sure she can deal with the TNT
I really don't see her coming back
but she likes the idea of you being in the back ground for the back up plan.
Here is a search engine on toxic relationships
Toxic relationships - Yahoo! Search Results
If you give me your and her birthday month/date/year
I can see if I can give you more insight on this.