As others have already said, MSQ is key to all your progress. Dungeons and later zones of the game (including the city where you unlock DRK) are all locked behind your story progress.

On your specific question about feeling undergeared: you only really need gear to match the level of the content you're doing. If you're doing Lv25 quests with a Lv50 character, it's okay to be wearing Lv25 armour.