Lots of people are riding the I/E threshold. I'm wondering if part of the reason they can't decide is not because they're always floating around the center, but rather that they have moments they classify as introversion, and other, distinct moments they identify as extroversion.
If that's the case, then maybe what's happening is that your energy and interaction-style is being affected by other issues like depression and anxiety, like this:
1. Default: Extroversion; onset of depression turns you inward, making you more contemplative and isolated, something you identify as Introversion.
2. Default: Introversion; onset of depression makes you compensate by seeking out people, approval, affirmation, something you identify as Extroversion.
3. Default: Introversion; social anxiety makes you talk more and seek out validation, making you behave in an Extroverted fashion.
4. Default: Extroversion; social anxiety makes you avoid social risks and people, so you end up acting Introverted.