Lol, this is nowhere near too long - I try to be brief and fail spectacularly each post
I hate when people are facing the divorce threat, and especially when he BPD party is female. Sadly, I have to point out the sad truth that even though things may come out clear at the end of a custody battle, there is a lot of damage that can be done to the man in the process.
I recommend, even though you DO NOT say you want a divorce, talking privately to legal counsel of some sort, just to "get it on paper". You need to set some groundwork simply to remove those threats from your W from being so strong, and also, as a "I am calling your bluff" ability to enforce a boundary. Again, talking to an attorney is not starting divorce proceedings. It's simply trying to find legal boundaries you can enforce around you and your kids. Others on here, and even in a board I think that is geared towards discussing custody, have been in your shoes, and can chime in or PM you about it.
Claiming YOU are the cause of all problems is pretty much on the BPD greatest hits album. Since the disorder has a lot of shame avoidance in it, removing blame from themselves is a BIG underlying cause of any time they get upset. They simply can't reconcile being wrong with being an okay person, too. It's black and white - all good, or all bad. If I make a mistake, ever, I am all bad. So it must by ocramolop! Not me, can't be me!
Never be afraid of posting "too much". It may take some a while to read it all, but we're here to share, to vent a bit (but stop short of ruminating), and to see what others in similar situations have had work, what doesn't work, and work on getting ourselves on0kilter. Sometimes, getting our own reactions under control can go a long way to improving the relationship, even if the pwBPD makes no conscious effort. We have to change the rules of engagement. This, in turn, makes them reevaluate their own reactions.