When I heard those reports, my sense of it was--and this is just an opinion, not very educated at that--he was disappointed about not being able to get in. Drugged? I guess that might be possible too. I've heard some suicide bombers drug themselves either for false courage or to lessen the pain.
Yes, it could be that he was sitting there trying to come up with Plan B.
Some people described him as sitting on the bench at first removing things from his backpack and putting them in his jacket or vest. Then there were reports that he was sitting there with his head down, apparently so still that he seemed to be asleep.
However, if the bomb was detonated remotely (and the detonating cell phone was not on his person), that changes things. I think the Muslims often do drug their bomb carriers, btw.