Sometimes I wonder if I'm just latching on to her diagnosis as a way to cope for myself.
this is an insightful observation. it's a thing i've wondered about myself and i'd wager lots of other posters have too.
however, don't discount
the diagnosis. if she really does have BPD, and doesn't accept that and won't go into long therapy for herself (because it is possible to ameliorate the behaviors that way), she will be incapable of a genuinely interpersonal emotional life and there would be little you, or anyone, could do. for example,
She does not seem to accept her diagnosis and blames it all on me
is very characteristic of BPD and in any case is a bad sign. it's hard to let go when you've been made "responsible" but it's right for you to focus on yourself.