I have never had imaginary friends. But I can help you tell the difference between happy/ manic and sad/ depressed. Happiness doesn't affect you in a bad way and is usually a response to something good happening in your life. Or if you are just generally a happy person. Mania has bad things happening like overspending, random sex with people, reckless behavior or driving, impulsive behavior, and even psychosis which is definitely NOT a symptom of just plain happiness and even normal at all. Sad is the same way. If you are sad because of a loss that is perfectly normal. If it affects your life in a negative way and lasts a long time then its abnormal. You may become suicidal if you are truly depressed. You can be disabled from having true depression or mania. Happiness or sadness shouldn't be disabling.