Differentiation in BPD and Choice Behavior?

Hey guys, I wanted to know how you guys or how psychologists would differentiate people's BPD from their natural behavior or vice-versa? Is there a cause behind BPD, or is it naturally occurring? Such a thing as BPD lite?

To expand, my girlfriend of six years and I broke up and she told me that during the relationship that my emotions were like a roller coaster, and that I would have moods where we would be really good, and then I would not interact with her very much, but then we would make up again. In retrospect I see that she was right. I constantly contacted her, and fell apart with delusions of how to fix things and it just made things between us worse. Since I realized I hurt her very deeply with my behavior and a lot of emotional immaturity, and the fact that she is not coming back, it has been very difficult for the past 5 months. I feel guilty for what all I put her through, and not being what she wanted. I know she thinks that some it may have been a waste of her teenage years (we are 20), and she seems depressed according to her blog. I think she found a new guy.

I am a very sensitive person but inwardly so. My moods are unstable more so than before (I assume due to the breakup). I can control my moods inward usually, and I guess I hide them from other people. This might be why people do not understand when I try to explain it to them. I also notice sometimes when I devalue a person just because of something small they said or how they act and then my opinion of them returns to normal. My point lies here: I am typically a loner and stay in my apartment room for most of the day outside of going to classes in college. I do not socialize much since I am content by myself and don't really want to form relationships where I can hurt people (like my girlfriend) or where I will end up looking like an idiot and people will eventually dislike me. I am not necessarily shy, but someone said this is a sign of fear of abandonment, which I can kind of see.

I consciously know and am aware of why I choose to be alone, and I tend to just shut off my emotions when I get too moody or things are overwhelming. I do not know if this is considered an empty feeling. I do not know why my moods are all over the place; however Vitamin D seems to help mildly. I do not know if these traits I have chosen are me, or if this is indicative of BPD, or what is wrong with me. I do not care about the label. It does not really matter. I just want to be more "normal," and get over my ex-girlfriend, socialize, and be a better person.

Anyone have ideas one what differentiates BPD, and normal behavior? I can associate with a lot of what many of you post, but I do not know what is with me. I feel like an idiot. I apologize for the lengthy post, and I don't mean to sound like a robot, but it is difficult to explain what I mean.