I know I can't change what has happened and there is no going back but i can't help but wonder... . Would she have stayed and dropped this other person if i had complied to her every wish?
Would you really wanted to walk on eggshells and always wonder what she was doing while you were away or getting on with your life too.
Co-dependency is all about compliance and negating your needs - have a good think scuba about that means.
Would he have always been there as a backup? Do they start the mirroring phase when they start hanging out as friends or when the actually consummate the relationship?
Always? who knows. Possibly or it could be someone else.
Mirroring is not a BPD term - everyone mirrors - you mirrored each other scuba thats how you got so enthralled.
_____hit
Rather than delve into what she may or may not do scuba start working on why you want to know and why you are more interested in her than looking at you. The answers are not with her.