A B737-800 Captain reported fatigue during a return flight from a foreign destination following a divert for weather; fog; clearance difficulties and weight and balance issues.

Narrative

[There was] weather en route and at our foreign destination; [so we] held [and then had to] divert for fuel. [We then] flew to the destination. Conferring with the First Officer; we both felt we were OK to fly back to our U.S. destination. Then; after brake release; we had problems with our clearance; loads; fog rolled in; ground operations came to a crawl; and planes backed up. By the time we took off; the sun was rising. We worked our way around weather in climb and initially en route. Once we were clear of all the challenges that kept us 'pumped up' fatigue slammed into us. I have flown this sequence 3 times previously in the last week; back to back; and I have three more; back to back. I knew fatigue was a factor under normal/adverse conditions. I should have called in fatigued when we got the turn to our destination from the divert station. I should have returned to the gate and called in fatigued when we were delayed on the ground.Honest reflection: I failed to project 'fatigue' factor. I knew from past flights fatigue would be a factor without throwing in a diversion. Though the First Officer and I felt we were fine at the gate; I failed to project the impact of fatigue. The First Officer had never been to this foreign airport and had never flown this sequence.Recommendation: 1) Emphasis projection of 'fatigue.' You may not 'feel that fatigued' at the gate; especially flying all night. 2) Eliminate this turn. Between the single runway operations; weather; holding; high altitude; 737 performance; flying all night with a 2 pilot crew; etc.; 90% of the time it doesn't work as scheduled. It may be 'legal' on paper; but it isn't 'smart.'