Mr. Jealousy(1998)

After his first date at age 15 ended with the girl making out with another man at a party, aspiring writer Lester Grimm has treated all his girlfriends with jealousy and suspicion. While dating Ramona Ray, paranoia gets the best of him when he discovers that her most recent ex is successful novelist Dashiell Frank. Lester begins attending the same group therapy sessions as Dashiell to learn about Ramona's past with him.