Welcome calcal01,
as VoC said - this is a big place and it takes a while to wrap your head around it. You are certainly not alone and a good way to ease your burden is to write about it. Another option may well be to get a therapist for yourself - you are exhausted, possibly slightly depressed (a lot of members are) and could benefit from having f2f backup in your corner .
BPD is multi-factored which means there are multiple contributors, not an infinite number but a couple. When it comes to symptoms the spectrum of stuff one can see is huge. Some of what you observe could well be related to BPD, some of what you describe indicate narcissistic traits and some of what you describe could be impact of living circumstances and changes. We don't diagnose here but what it is likely that discussing strategies here on the board can benefit you.
On the other hand I feel he is manipulating and selfish, with no real solid grasp of empathy gratitude or compassion.
He interrogates me when he starts to "loose" an argument. He starts questioning me over and over again to no end until I get befuddled and cry. He has recently admitted to using interrogation techniques in fights to manipulate and get his way, not only with me but with people at work.
Sounds like he has experience to put a lot of pressure on people. He certainly has no right to behave like he does. Asking questions is one way to assume a position of superiority and power. In a relationship of equals that should not have a place . Not playing his way is the only way to not win but limit the impact of him stepping over the line. A study of our boundary workshops
and then leaning on the board could help you changing your behavior here. He may be behaving out of bounds but
in the end you only control yourself. That sounds not like much - it is however the key to maximize your impact on your relationship.
Again

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a0