Is it at all possible that cannabis could ever cause you to feel different mentally forever after smoking it?

Unfortunately - YES.: As a psychiatrist, I consider potential long-term risk of Cannabis (panic attacks, anxiety disorders, paranoia, dissociation, psychosis, aggression, ataxia, hallucinations, illusions) to be of significant concern.

Some evidence: Some studies have concluded that smoking cannabis makes people more susceptible to psychosis and paranoia. Cannabis clearly has an effect on the developing brain as seen with teenagers who smoke.