Hi and welcome!
You have a lot of work ahead of you... .!
Firstly, tell us more about your situation: roughly how old are you both, how long have you been together, do you have kids? She has been diagnosed, is she getting treatment?
It's great that YOU are seeing a therapist. This will help.
She says that I need to change or she can't be with me because I am "killing" her, but I am not sure anymore if this is just part of the disorder, or if I really am a terrible person.
This is the disorder. She will blame everyone and everything else for all her feelings and behaviours. The emotional emeshment of BPD makes partners question their own worth, sanity and, well, everything!
She doesn't make any affirmative moves to kick me out, or to leave herself, and I think deep down she is afraid of me leaving (even though I never would). Is this common with BPD (she hasn't started treatment yet)?
The BPD's biggest fear is of abandonment. She has low self-worth, and cannot understand why you are with her. She will question that until she dies. And no amount of reassurance, love or commitment will EVERY fill that hole of hers.
I know I have my own self-improvement that I need to do, there are ways that I have failed her in all of this, and probably some ways that I contributed to this condition (I was her first romantic partner)
You did not contribute to her BPD. You possibly contribute to her anxiety (perhaps 5% at most) by failing to understand her, and you probably contribute nothing to her panic - but perhaps with education you can help her to reduce it.
Living with a BPD partner is hard - you've probably found that already. But you can. You cannot change what she feels, you cannot "fix her", all you can do is understand her and be the rock in her life. It requires strength of character, a thick skin, and above all else, self-confidence and patience. This will be a long journey, but what you learn will help your relationship, and actually help develop YOU as a person as well.
There is a lot of material on this site to read. Check out the green sidebar first ->>. Start learning about BPD - a good book to read is "Walking on Eggshells".
And post here lots!