hi SoMadSoSad
yes, this is one of the hardest things to deal with, and its not easily understood. it really does a number on you. i remember being in a pretty dark place, asking my friends and family "does she even THINK of me?".
there are some answers pertinent to the disorder that make this no less painful, but easier to understand.
1. people with BPD form insecure attachments. suffice to say they do not attach in a "normal" way, nor do they detach in a "normal" way. in general, they do not experience the relationship in the same way that we do.
2. people with BPD often have finely honed compartmentalization skills.
3. people with BPD have issues with object permanence. the memories, the feelings associated with them, are hard to recall when we are not in sight.
4. it may depend on the circumstances of the breakup. if shame is involved, there are all sorts of defense mechanisms that protect against feeling it.
5. its not always true that they never look back. my ex gave me this impression, then i discovered she was getting into my email late at night. thats not always the case, and it did not mean my ex was having second thoughts.
suffice to say, what gives us this impression, and the extent to which it is reality, is all based on finely honed defense mechanisms. again, this makes this no less painful, but easier to understand, and learning about the disorder helps to depersonalize the painful actions.
have a look at Susan Anderson's five stages of abandonment here:
https://bpdfamily.com/message_board/index.php?topic=154676.0where do you see yourself in these five stages?