Fear of hell can persist long after you've intellectually rejected religious beliefs because these fears were often instilled in childhood when your brain was most impressionable. This is sometimes called 'religious trauma' and can cause real anxiety and distress.
Remember that this fear was taught to you, not discovered by you. Many people find that therapy, particularly with someone who understands religious trauma, can help process these fears. The anxiety typically diminishes over time as you build new beliefs and experiences.