I can't stop thinking about my loved ones dying

I (F21) am going through some pretty stressful times. A loved one has an upcoming medical examination which is partly to test for cancer. The doctor has reassured us that this is to rule out the most serious possibility based on her symptoms (she is only presenting one or two that match) instead of to confirm a strong suspicion, and even if she were to have it, it would likely be the kind that can be treated well. I know all this but the situation has left me in shock and struggling to process things.

I now cannot stop thinking about people in my life dying, including ones who are not exhibiting symptoms of any illnesses. The anticipation of the examination day has left me so anxious that I'm unable to focus on anything else. I struggle to focus on my usual hobbies like playing guitar and reading, I'm not sleeping well, and the recent heatwave in my country has made it hard to get into nature, which usually calms me. I also worry that by constantly catastrophizing about the worst case scenarios that I am somehow making them more likely to happen: the more logical part of my mind knows this is not possible but it is hard to reassure myself when my nerves are so frayed. I know there is a very good chance things will turn out fine and my loved one suspects her problem is muscle related, but I often catch myself idly thinking about the way she (and other people I love) could die before having to remind myself that they are very much alive.

These constant existential thoughts about mortality and the briefness of life make it hard for me to get on with necessary activities like making dinner and doing laundry, but I'm trying. I'm seeing a therapist for the mean time to try and gain some perspective. Soon, I will hopefully know more about what my loved one's problem is and have a clearer idea of what the future looks like, because this forced present living is unbearable. I know that even if the worst news is given to me I will still get up each morning and go to bed each night, what other choice do I have? I'm not looking for anyone to try and fix or diagnose the problem for us, but any sympathy or advice on how to regulate my racing mind would be appreciated.