If you're out of healing items, than you might just be stuck.... I'm guessing you're playing on casual with the ability to save INSIDE the dungeon?
The Sewers of Horrors is designed to be the toughest part of the game, brutally tough so it's not that you "suck" so much as the area is terrifying.
The Sewers is still relatively new but it looks like party supporting Clerics have the easiest time because of their healing and party boosting skills (Arbenos' Might and Fire Plane have an obvious challenge). Warriors, all three builds, seem to do well so long their healing potions stay in stock (healing Hirge helps, too). Rogues and Mages have the hardest time, on average, because their glass cannon natures are inherently challenged by marathon dungeons. There are builds for all three classes that dominate the Sewers, but it is designed to be a tough area.