Cool idea for a Sixth Sense-like supernatural thriller! You've got two intriguing hooks: the mystery of why his patients all died on the same day, and travelling back in time to confront a twenty year old secret.
I think it needs to be clearer in the logline how the ghost and time travel ideas are related, because at the moment it sounds like pitches for two different movies. Perhaps he travels back in time to find out why they all died, and discovers he is personally linked to their deaths?
Also, be careful with "travels back to his past", because I'm not sure if it means time travel, or just revisiting his past. Maybe "travels back in time" is clearer.
Here's my attempt:
When a senior psychologist discovers his patients are all ghosts who died on the same day, he must travel back in time to find out why, confronting his dark past.
- Judge, Patrockable