Psychosis is a temporary condition, not a mental illness. It is something that generally happens as a result of another illness and, sometimes, certain medications, etc. It's more a symptom. It's like having trouble breathing when you have pneumonia. That's why your original dx was major depressive with psychosis. Think of it like you would a panic attack of sorts.

If you "normally" spend too much time in your head (I did) that's probably a bipolar, depression, personality, or anxiety disorder. But a really bad episode where you are totally out of touch, would be psychosis.

If you are not "just" in your head but are hallucinating or overly paranoid or feel you are being watched or controlled by others or that you are someone else besides who those around you know you as, etc. that is more schizophrenic or psychotic. Yes, people who are psychotic have psychosis :-) but it is only if that is "all" or the main symptom that they are psychotic. Someone with active psychosis would not be likely to take the Sanity Quiz, would be concerned about too many other things.