Hi Mrparsnip and welcome to the boards
BPD relationships can be high emotional intensity... not just for the pwBPD, but for you, as well. I really hear your confusion and struggles, and the emotion going on for you right now. It must be a lot to carry, plus the uncertainty and fear about losing the relationship. That would grind anyone down.
pwBPD really struggle to manage their own harmfully intense emotions. They typically don't have as much bandwidth for their partners' emotional struggles. It's kind of like if you were trying to carry a full 5 gallon bucket of water with no lid, and someone comes up and is like, "Can I just pour a tiny bit of water in here? Could you carry that for me? It's like one drop, it's not that much." -- you literally have no space for it on top of your own struggle to carry your full bucket.
My thought about this:
5 days ago I gave in. I sent her a heartfelt message about how much I loved her, how much I.wanted it to work and I would work hard to support her and make the relationship work. No reply........
I'm devastated..... I know she has said a lot of these things before and come back, but this time around I think this may be final and she has never not replied to a message... no matter how emotional or upset she has been ever.
is that dynamic (full bucket of water) may be going on for her. pwBPD might get more easily overwhelmed that most people at even positive emotions. We think we're offering love, support, and devotion -- even though those are positive things, sending a lot of messages like that might overwhelm her, as she's dealing with her own stuff.
While there are no guarantees in life, it is something that she was interacting with you over the shorter messages.
She may need a longer time to get back to an emotional baseline than she did for those messages (and longer than non-pwBPD need).
I might play it cool (not cold/icy, but low-key/chill) for now. Don't press her, don't chase, don't pressure, don't let your own anxiety drive you to pester her for reassurance.
If/when she reaches out again, keep the content low-key, easy, no emotional strings (Does she like topics like animal rescue videos? Puppy training? "Good news" reels/links?). Steer away from "relationship talk" (unless she explicitly wants to go ther) or "declarations of your love". Allow her to drive the pace/content of interactions. Avoid "needy" moves ("can't you just tell me if you want to be together or not?", "I've missed us so much").
Most importantly -- even more than "will she reach out, will we reconnect" -- is you working on yourself. The relationship dynamic you two had didn't get there overnight, and while part of her contribution was BPD, you also contributed to the dynamic. Figuring out what your contribution was, will give you a better chance of steering clear of this breakup/makeup cycle in the future.
One place that might help to start, as you wait, is learning about the
Karpman Drama Triangle. It's a framework (there are many) for understanding how we participate in conflict in unhealthy ways. If you decide to take a look at the link, let us know what stands out to you there.
Hang in there;
kells76