Hello, and welcome, RiverAce
Perhaps I should say welcome back? Whatever, we're here to support you.
What you describe in your fiance sure sounds like BPD traits, whether she is at a diagnosable level or not. So I think you will find the tools here helpful for you in coping with her and understanding her, no matter what you choose.
Let me ask a couple more questions to help sort out your situation.
Do you currently live with her?
You say you are engaged. Is there a wedding date? And if so, is it coming up soon enough that you need to call it off asap if you have second thoughts?
I've also got a very practical suggestion for you, which can immediately improve your relationship.
You describe shouting matches. You describe her being set off accidentally, and this:
there has been a distinctive pattern where I’ve changed, where I’m not measuring up, where I’m not ambitious enough, where I’ve withheld information from her, where I don’t take of issues in a timely manner, and the list goes on. I react very strongly to her invectives as I think she’s attacking my character (a button that’s easily found).
In ALL these cases, as soon as you realize things are going badly, take a time out. Tell her you can't discuss it now. Leave the room if you need to. Even leave the house if she follows you through the house. Sometimes just going to the bathroom (and letting yourself think without being attacked for a couple minutes) gives you space to decide what to do.
For more on how to do this (and why), read this workshop:
How to take a time outOnce it turns into a shouting match, you are going to say things that hurt her, and she is going to say things that hurt you. First you stop cutting on each other, then you try to bandage yourself up. (And let her take care of herself, as if you get back together too soon, you will go back to hurting each other!)
One more tip--if you (or she) gets worked up, it takes 15~20 minutes for the adrenaline to work its way out of your system if you don't do anything new to restart it. Taking space for that long at a minimum is a good idea.
Take good care of yourself! Do you have good friends or family you can talk to nearby? If not to deal with this insanity, just to get some time away from it?