I feel like I don't have a "good enough" reason to leave her.
This is the story of my past 3 months.
I've read your post. I am struck by a couple things- your gf may, in fact be a rare case- she may be plowing through all of the steps to achieve wholeness by undoing the traps of her past traumas. Given she is only in her early 20's, I find this VERY hard to believe. I don't think this is possible.
And you don't either- by saying that there is a real possibility that she will decompensate if you leave her, you are, in fact, saying that you believe the changes she is experiencing in her world are paper thin and and sturdy as spider webs. The progress she is making- as a 22 year old woman in therapy and group sessions- is only as good as her ability to mirror what people expect of her. An abandonment will - with certainty - cause her to crash down and show her true colors to you again.
If your heart is with someone else now, you are not being fair to anyone. You are being absolutely cruel to your BPDgf, by stringing her along, making her jump through hoops to be "better for you." If you want to be with the other woman, be a man, and do it- cut your losses, and get out.
If the other woman is not worth the hurt you'd cause, then be a man, and cut it off with the other woman. Avoid her at all costs. Convince yourself she is ridden with herpes, chlamydia, and HIV. Do anything to remove the temptation.
It sounds like you want cake and eat it too. Make a choice and believe in it. It's MUCH harder to do anything when you realize you haven't gotten your cake, despite 25 years of marriage and 2 kids to raise.
Surg_Bear