Welcome, ConfusedBF!

I wanted to take a moment to welcome you to the discussion forums. I'm so sorry for what you're going through but glad you have found a community where many of us have been through similar experiences, and we can learn from each other. From what you have written, it seems clear you will get good ideas and support here if you continue to read and post. In short, you have found the best place in the world for understanding, compassion, and education as it relates to coping with loved ones who have personality disorders.
Thank you for sharing with us what you have thus far:
Bottom line is... .I don't know what to do. I love her deeply and want to see her come through this. I want be there for her and hopefully share a life with the woman I spent the last 4 years with. Knowing about BPD and feeling strongly that this is something she struggles with actually gives me hope. I wish I had known about this 4 years ago, it would have made so much more sense why she is the way she is sometimes.
I just have to applaud you for being willing to continue to put forth effort into understanding your SO. In my opinion, that's true love. This site is rather HUGE, however, there are many, many articles related to bettering your relationship with the angle of using healthy communication techniques, setting needed boundaries, and validating what is valid. You will find these articles in the right-handed panel on this board. We can help you with questions, point you in the direction of additional resources, or just be a sounding board.
Should I keep my distance? How do I know when to attempt to draw her near again.
I know that it can feel quite maddening to not know the answers to these two questions. However, in time, only YOU will know how best to answer them. We don't give run or stay advice here.
How do I cope with this lonely feeling that overwhelms me whenever she pulls away and pushes me away.
Tune in here, and be amazed. I know you are here by way of sorrow, but I just want you to know that you are among peers, friend. Unfortunately, there are legions of us! I'm so sorry you are experiencing this. However, I think you'll find a lot of parallels here - lots of members (including me) have similar stories. You will see from reading the posts here that you are
far from alone.
Tell us more about your story. It helps you to get it out, and it helps others when they see that they are not the only ones suffering. We look forward to hearing more from you.
Keep writing, keep processing, keep learning!
-Speck