Hi beardedgiant,
It sounds like you have been through a lot in a very short period of time. I'm sorry you've had such a difficult time.
I hope you keep coming back and keep reading and posting here because it has helped me more than I can possibly say in this short post.
My story isn't very different from yours. When I met my partner I couldn't believe how perfect we were for each other. She liked all the same things I liked, laughed at all my jokes and seemed to think I was the greatest thing ever. I was on cloud nine for about 4 months.
Very quickly there were red flags that

I didn't understand, odd mood swings over things that didn't make a great deal of sense
to me. One of our first arguments was about me taking a bike ride with my cousin on a day when she was at work. To my SO it felt like my attention had shifted away from her and I didn't love her anymore. To me it was exactly like you said
It was so unexpected, I had never met anyone with BPD before ... .I was just completely blindsided. I was overwhelmed by the experience
Because I didn't understand and it felt so foreign to me I yelled back, which made the situation worse.
Until I tumbled on BPD and this website we went through some very very bad times. Horrible arguments and we spent about a year apart letting the emotional temperature settle down between us.
Now we are back together and doing (for us) very well. We are fortunate that my SO is very committed to therapy, religious about her medication and dedicated to her reading and self improvement. She very carefully never says BPD to me and I am very careful to always use words like we and us when we talk about r/s issues. I did about 2 years of therapy to understand my role in this dysfunctional dance.
Things are better than I could have ever hoped for between us. We have frequent success in handling what used to be very problematic issues. It is not easy. By no stretch of the imagination could this be called a 'normal' relationship. Whatever that is.
To a very large degree there are three of us in this relationship, me, her and the disorder. Most days I am reacting to the disorder first. I suspect it will always be that way.
I think you are doing a great job trying to sort this out. It's a tough decision. I hope you took some time to look at the lessons on the right side of the page. Specially number 2 Understanding your role in the relationship. because for me that's where the rubber hits the road. Well okay maybe in number 4 too.

Best of luck. Keep reading and please come back and let us know how you are doing.
'ducks