extreme anxiety

About three weeks ago, I started really worrying about death/ the world coming to an end. I actually had a panic attack about the sun imploring and all of humanity being snuffed out and the complete and terrifying nothingness after that happens. Since that day I can't shake the panic of nonexistence. It keeps me up at night and sometimes interferes with my daily life. I know that death is inevitable, I also know that worrying about it constantly is not going to help anything. I can't stop. I cut caffeine out of my diet, because I heard that that helps relieve anxiety in some cases, but it hasn't helped. I'be always been a bit of an overthinker, but this is awful and I'm not sure how to handle it. Please help...