How do i know if i'm really mentally ill or if just all in my head?

Get evaluated: You could see a phd level psychologist for a psychological evaluation or a psychiatrist for a psychiatric evaluation. You could also see a social worker working at a mental health center that has familiarity with the diagnosis of mental illness. If you are struggling with psychotic thoughts or thoughts of suicide or homicide then you can be evaluated in the emergency department. Stay safe!

See doctor: It was said in my good old psychiatry textbook that 67% of population need psychiatric help from time to time. This does not mean however that more than 2/3 of population are truly mentally ill. It may be stress, mood disorder, personality disorder, wrong perceptions, misunderstanding, etc. Go to your doctor (not necessarily to psychiatrist) and sort it out.