Hi ReadytoRun,
Welcome. I am sorry that you are going through such a difficult situation. Feeling like you are on a roller coaster ride is certainly not fun. I understand how confusing the behavior is and how it can give you anxiety.
The behaviors you described are some common characteristics of BPD. People with BPD tend to socially isolate themselves, withdrawl, and avoid things. It is a coping mechanism used as a reaction to emotional situations and emotional states. Essentially, a pwBPD has a very hard time regulating their emotions. The majority of their emotions are internalized feelings of sadness, shame, and anger. This type of learned behavior stems from their childhood. It becomes learned and then repetitive maladaptive behavior.
Here is an article that provides more information about BPD.
BPD: What is it? How can I tell?Learning how to effectively communicate with a pwBPD can make a world of difference. We have many tools on this site to help you communicate with your SO. Take a look at the lessons on the right hand side of this page.
Although understanding the behavior of BPD is important, it is also important to take care of yourself. Have you had the chance to focus on yourself? I found having a "support team" of family members, friends, and my therapist really helps me cope with the erratic behaviors of my pwBPD.  :)o you have support outside of your relationship?