What Christina said, above.

Mood lability (rapid ups and downs) is one of the hallmarks of borderline, although there is such a thing as an "ultradian" bipolar pattern with multiple mood changes in a period of days or even hours. It's fairly rare in BP though.

Since you haven't really been diagnosed with anything, we can't offer much except our own experiences. People tend to diagnose themselves off the Internet, and it really drives doctors up the wall because the average layperson will get just enough knowledge to be dangerous.

I know you feel awkward about talking to a doctor. I did too. I was relatively sure I had bipolar, but I was mortified when I first started seeing a psychiatrist, because I thought it meant I was crazy. But you must go in order to find out what's really wrong with you and start treatment. Having a mental health diagnosis puts you at a much higher risk for other medical problems, and you want to get a handle on the illness so you don't run into these at an early age, as well enabling you to live your best life.