Hi RandB34-
And welcome! I'm sorry you've been through all that, it is very painful and confusing, although not unique around here, we understand.
My question is, was she always that disordered but pretending to be normal?
Order becomes disorder in the first few years of life, and since she's been diagnosed with the disorder, yes, she's always been disordered. Although "normal" is negotiable right? Borderlines need to attach to others emotionally, it's the main focus, so being who she needed to be to attach to you and others was the priority, still is.
Why the sudden change after the diagnosis?
There's no way to know without knowing her, but my guess is she learned about some of the traits and behaviors of someone with the disorder, they might have been things she always wanted to do but couldn't or wouldn't, and being diagnosed validated her, gave her permission, to do what she'd been thinking about anyway.
What did she mean by "the love was real" and she learned all she could from me?
Borderlines feel emotions intensely, so when she loved you she loved you all the way, deeply, you were her knight in shining armor. But relationships with borderlines are also inherently unstable, so when she didn't love you anymore, she didn't, all the way. BPD is a shame-based disorder too, so telling you she learned all she could from you could have been her way of justifying leaving you, by framing it as something like "everyone in my life is just here to teach me something, and once I've learned it, I'm off to a new teacher." Sounds cruel and cold, although the alternative would be face the shame of causing you pain and take responsibility for her behavior, which would have likely been far too painful.
Was she using me? Not once did she ever ask me for any money.
You would know how real it felt in the beginning, and it was for her, until it wasn't. Borderlines are not fundamentally malicious, she's just responding to her own strong emotions, and feelings are facts for borderlines, and depending on the severity of the disorder it's more like responding to the emotions of the moment rather than having a long-term plan.
There's stuff for starters, keep reading and posting, it gets far better, and take care of you!