Can't obtain poison skill

Related to the quest to obtain poison skill as rogue;

I started out by finishing the quest given by Rogue Trainer Shenthul in orgrimmar
to the point were i was told to travel to hillsbrad in order to turn in the last part of the
quest. I went to hillsbrad, and was not able to finish the quest in, after several attempts
to fix the issue i decided to abandon the quest in order to start it over again. As it turns out,
Shenthul does not have the quest available for me anymore and thus i can't obtain the poison
skill.