My mother died suddenly of cardiac arrest when I was 23, she was 54, and I had lots of health anxiety. What I learned in AA was to a) stop projecting and keep reeling my mind back into today, b) realize the fear was really about something else (from my childhood), c) therapy and helping others. As an alcoholic (27 years in recovery) I had self-centered fear. I realized I'm not only powerless over alcohol, I'm also powerless over people, places and things.
What can be an huge help is meditation. There are dozens of free guided meditation apps you can download. Start with ten minutes per day and increase. Also prayer is a big help, as is keeping a journal.
Realize that this is a temporary situation and it will pass!