How do I handle the fear of being responsible for lives in command?

That fear is the right response to what you now hold. Lives depend on your judgment, and if that did not frighten you, something would be wrong. The terror you feel is your conscience taking the job seriously. Do not read it as proof you are unfit. Read it as proof you understand. You will not keep everyone safe by force of worry. You keep them safe by preparation, by clear thinking, and by trusting the training you and they have done. Lean on your people. Command is not one person carrying everything alone. It is a group, and you are its head, not its whole strength. Turn the fear into care. Let it make you thorough, not frozen. Learn your people, know your plans, and then act with steadiness even when you are unsure inside. They do not need you fearless. They need you present and clear. That you can give from the first day.