You are such a soft hearted soul!
I understand your panic when your ex started referring to a final goodbye, but I think (and please forgive me for being blunt) by swooping in and rescuing her, you did not do her a favour - quite the opposite, in fact.
Your ex knows what you are like. She knows that it doesn't matter what she does, she can rely on you to be there to catch her when she falls. And herein lies the problem: she is still not taking responsibility for her own actions. If she doesn't own her behaviour and make active steps herself to correct it, it just continues.
Again and again, she will feel bad, she will call you, you will help her, she will see if you have changed your mind regarding your ultimatums and when you haven't...she will wait until she is strong enough to stand up on her own two feet and then fling you away again.
You'll be left there, feeling devastated, because again you have put so much into helping her and it has made next to no difference. Your own emotional life will be in disarray and she will continue on regardless, until she needs you again.
I firmly believe now that the key to handling someone with BPD is setting strong boundaries. We are damaged children inside and like children, need firm boundaries and rules to guide our behaviour - a 'black and white' set of instructions that override the emotional storm. Even if we fight against the rules at first, acting out and rebelling because the adult side of us will fight for autonomy (like your ex, now, because you put your foot down about the alcoholism) there is safety and security in having the rules there too.
Think of it as an anchor - BPD people feel lost, disempowered, like we are at the mercy of the emotional tidal wave with nothing to hold onto.
We have to have something to hold onto and believe in, which is why we cling so desperately to others to avoid floating away.
I firmly believe you need to be steadfast in refusing to 'rescue' her until she takes the first steps to helping herself. She knows where to go and what to do...now she just has to do it!
The sad truth is, she may choose to take her own life. Many BPD people do, as high as 10%, I believe. But that can happen regardless of whether you are there or not, L - it's just about the only decision in this world that you make completely selfishly. But we are damaged people already, L - long before you came along.
You have shown her compassion, understanding and guidance - you need to let her decide what to do now.
Anything else and you are doing this for YOU, not her. I know helping makes you feel wanted and validated as a human being and gives you a sense of purpose...but, in the long run, it isn't help. It's a flimsy band aid covering a much larger wound.
Be the anchor, L. Weather the storm but stand fast.