That was hard for me at first, too. What really helped me is keeping a mood chart. That way you can see how often your moods fluctuate and how long they last, whether they're tied to changes in sleep, etc. You can also notice when you're feeling more stable and keep track of your baseline personality during that time.

Since I'm bipolar, my mood episodes tend to last for longer periods of time and have a big effect on my sleep. For example, I sleep way more (or the opposite, I get insomnia) when depressed, and need less sleep (but feel way more energetic) when hypomanic. I can often see the changes in sleep setting in before I consciously register the mood shift. My episodes also often (but not always) happen without any clear cause, e.g. I can get depressed just because my brain decides it's time for that, not because anything bad happened. Sometimes my mood fluctuates faster, such as during mixed episodes, and of course there are the same daily fluctuations that everyone experiences, but overall I can see pretty recognizable long-term patterns.

To my understanding, with borderline you would expect more erratic and faster fluctuations in mood that are often an overreaction to something in the environment or relationships. There are also other factors that would distinguish borderline from bipolar, such as fear of abandonement (which is a key characteristic), "splitting" (black and white thinking), etc.

Keeping a mood chart really helped (and still helps) me to figure out what was going on. I personally like this mood chart, maybe it will help you as well: http://www.cqaimh.org/pdf/tool_edu_moodchart.pdf

As for hypersexuality as a coping mechanism during depression, I definitely think that's possible. I experience it during hypomania, but to complicate matters it sometimes also happens during depression. The underlying reasons and feelings are different, though. Again, I find mood charts or journals useful in figuring out what's behind things like hypersexuality, spending sprees, etc.

As others have mentioned, I think it would be a great idea to get a professional opinion, and things like mood charts help with that because thinking about it beforehand makes it easier to convey to your psychiatrist/therapist what your issues are.