Doctor Answers (3) on Mental instability
Check yourself. You may come with a reason. And if you want to overcome this and it's disturbing your day to day activities do consult a psychologist and handle it better.
If what you are telling is mood instability in which sometimes for few weeks you are sad, disinterested and have low energy while sometimes for few weeks you are more energetic and have racing thoughts with elevated mood you may be suffering from a mood disorder. If there is a family history of mental illness there are more chances of having a mood disorder. Having said that not all happiness or sadness is a mental disorder. It's important to get a detailed evaluation with a psychiatrist that will help clear the diagnosis.