Mark,
I feel the same exact way, and your story is nearly identical to mine, except the timing differences and the number of children.
Cat has some great advice, and has neatly summarized it; however, if you want to do a deep dive, in addition to the tools that she mentioned read on...
I am going to recommend two books, one to get your head around the illogical 'logic' of what is happening, and the other to see your own role as a 'caretaker' and what to do about it.
“Stop Walking on Eggshells” by Paul T. Mason and Randi Kreger [3rd Edition]
“Stop Caretaking the Borderline Or Narcissist: How to End the Drama and Get on with Life” by Margalis Fjelstad
When I first read these books, the concepts were very foreign, like a foreign language; however, they will eventually become crystal clear [took me half a year] after learning about it.
I will wrap this up, with my number one piece of advice, and that is to do 'self-care' - make sure it includes individual therapy [for yourself], exercise outdoors [as simple as a walk], among other activities that you enjoy doing to recharge your spirit.
If you have any questions, please feel free to ask, and take care.
SD
P.S. Another great section to learn about being with a person with BPD [pwBPD] is:
https://bpdfamily.com/message_board/index.php?topic=329744.0