Scared I could die any minute especially in my sleep

I had covid in the beginning of Dec and still dealing with harsh side effects on top of that I just got diagnosed with cancer. I have a tumor the size of a grapefruit in my chest and I’m in severe pain all the time. The anxiety coming with every new symptom is debilitating. Dr said I can’t do anything for pain because chemotherapy is the only treatment for my case. The tumor is pressing on my heart and lungs and I’m scared I’m just going to stop breathing at night. It may take another week or two before starting chemo. Any advice please???