What are the signs of serious mental illness?

Impairment. There are so many types of psychiatric disorders, it's hard to offer a single set of descriptors of what's "serious." they have in common behaviors, thoughts, & feelings that recurrently interfere with your ability to go about your life. The more they interfere, the more serious they are, whether fears, depression, mania, psychosis, ocd, adhd, anxiety. You don't need to go it alone; seek help.