Hello Leiah, welcome to the group -- so glad you joined and are reaching out for support.
Many more people "lurk" (read without joining) here than are posting members, so while I can't think of any GP's with BPD adult GC's off the top of my head, I do know that (a) there are many grandparents who are members here, and (b) your post may give encouragement to any "lurkers" who are GP's coping with BPD GC -- perhaps they too will join.
5 years is a
long time to deal with BPD traits and behaviors. I get what you're saying about 5 years' worth of stress that's all held in, and how it can't go on like this -- something needs to change.
It seems like part of you knows that the living situation can't continue this way, and part of you has some fears about the change:
I want my granddaughter to succeed. I have provided a safe place for her to live while attending school or working until she can save up for a place of her own. Long story short, that's never going to happen as long as she has me as a safety net.
Without me, I don't know how she will ever survive.
I fear that call that she's been found seriously injured or worse.
I wonder if you recognize that while your support had its season, that time is drawing to a close -- yet you have fear about how your GD will cope... if at all.
One idea that gets talked about around here is the idea that on an airplane, "you have to put on your own oxygen mask before you help others with theirs". It's the concept that if we aren't taking care of ourselves -- if we're at a breaking point -- there's no way we can really help the others around us. So, it's OK to recognize that you have done all you can do, and to turn some of that care towards yourself.
I'm guessing that you and your GD's mom are not in much contact? Are there other family members with whom you do have a better/more supportive relationship? What about friends, coworkers, religious group members, etc -- how is your support system these days?
Strangely enough, for some people who parent adult children with BPD, when they start "doing less" for the pwBPD (person with BPD), that enables the pwBPD to build their own skills and use their own resources to make their own way in life -- like you mentioned, you have this insight that as long as you're the "safety net", your GD may not have the motivation to try on her own. While the choices that many pwBPD make to "strike out on their own" can be unconventional, worrisome, odd, or not what you would've wanted, those choices are a start and may reflect the best the person can do, given how BPD is a serious mental disorder.
So, while your GD may choose to live with friends, live in a van or a tent, not have permanent employment, etc, that may be the best she can do with her skills at the time. Living on her own and paying rent on her own might be beyond her emotional ability, yet if she's couchsurfing and has a temp job, that could be what success looks like for her, and could be a first step towards her taking more responsibility for solving her own problems.
Hopefully that is some helpful food for thought -- we truly look forward to hearing back from you. Let us know how you've been doing;
kells76