Dragging out relationships can be a trait of many disorders, or simply a maladaptive trait outside of any disorder. It's often indicative of low self-esteem, which can happen with most psychological problems, if only because our disordered functioning makes us feel insecure.

No one here can really tell you what your diagnosis should be. It's not unusual for people with bipolar to have moods triggered by outside events, but that also occurs in BPD and you really need a good psychologist or psychiatrist to help identify the underlying traits that will tell you what your real diagnosis is. It's also possible to have both a personality disorder and Bipolar. But if you don't trust what your doctor is telling you, I would always suggest that you seek out another opinion.