EMB ERJ 145 ER/LR
Air Traffic Control Fully Certified
Situational Awareness; Fatigue
ATC Issue All Types; Conflict Airborne Conflict
Air Traffic Control Issued New Clearance
Human Factors
I had descended the E145 to FL280 a few minutes later the Conflict Alert activated. I had assumed the E145 was inbound to ZZZ on the new arrival route that had just been implemented. However; the aircraft was moving to the south and head on with a B737 at FL290. Once I had realized the Conflict Alert was authentic; I told the E145 to expedite his descent and turned both aircraft 30 degrees right. At the same time; the B737 responded to a TCAS and climbed. As a result no text book definition of separation lost. I believe the cause of this situation is the prolonged levels time on position and of extensive workload; thence resulting in mental fatigue.
Enroute Controller described a loss of separation event claiming 'mental fatigue' was a contributing factor.