When you swing from one emotion to another, first feeling happy and then very sad for no reason, this is an example of emotional instability.
- lack of stability; unstableness; specif.,
- lack of firmness or steadiness
- lack of determination; irresolution
- tendency to behave or react violently or erratically: the instability of a chemical compound, emotional instability
(countable and uncountable, plural instabilities)
From Middle French instabilité, from Latin instabilitas