According to Wikipedia, "In his personal life, Sellers struggled with depression and insecurities. An enigmatic figure, he claimed to have no identity outside the roles that he played. His behavior was often erratic and compulsive, and he frequently clashed with his directors and co-stars, especially in the mid-1970s when his physical and mental health, together with his alcohol and drug problems, were at their worst...."
"When he appeared on The Muppet Show in 1978, a guest appearance which earned him an Emmy nomination for Outstanding Continuing or Single Performance by a Supporting Actor in Variety or Music, he chose not to appear as himself, instead appearing in a variety of costumes or accents. When Kermit the Frog told Sellers he could relax and be himself, Sellers replied: 'But that, you see, my dear Kermit, would be altogether impossible. I could never be myself... You see, there is no me. I do not exist... There used to be a me, but I had it surgically removed.'
--Peter Sellers,
The Muppet Show, February 1978"
"The stage play
Being Sellers premiered in Australia in 1998, three years after release of the biography by Roger Lewis,
The Life and Death of Peter Sellers. The play premiered in New York in December 2010. In 2004, the book was turned into an HBO film with the same title (The Life and Death of Peter Sellers), starring Geoffrey Rush.
The Belfast Telegraph notes how the film captured Sellers's 'life of drugs, drink, fast cars, and lots and lots of beautiful women.' Although the film was widely praised by critics, both Lord Snowdon and Britt Ekland were highly critical of the film and the enactment of Sellers; Ekland believed that the film left audiences with the wrong impression, saying 'the film leaves you with the impression that Peter Sellers was essentially a likeable man when in reality he was a monster. He may have been a brilliant actor, but as a human being, he had no saving graces at all...' "
In other words, because of Peter Sellers's enigmatic life, some say that the film doesn't exactly portray the real Peter Sellers, since the only person who really knew him best was Sellers himself, and it is seemingly impossible to fathom or comprehend his private life...
http://en.wikipedia.org/wiki/Peter_Sellers
I am honestly unable to give any qualified professional opinion of whether or not Peter Sellers might have also had schizophrenia....