Learning that your cancer has recurred is a nasty shock. Your response to it seems fairly normal to me, and I don't know that trying to make these quite reasonable feelings go away is something to put a lot of energy into. Adjusting to a different view of your future life will take some time, but with a little time you'll adapt to your new normal. As Denise mentioned, having a treatment plan will help too. Do you know anything about your prognosis?

After I was first treated for endometrial cancer (stage 1b, grade 3), I decided I was going to live my life as if it was never coming back, even though I knew there was a solid 50-50 chance it would be back. I got my chemo port removed; I took a new job. So when I learned it was back 9 months later, I was devastated that the future I was imagining for myself was gone. (The cancer was spread throughout my peritoneum, so while potentially treatable, this is considered incurable. People usually live a year or two.) I quickly went on Rocket Lawyer and wrote a will. I pestered my foot-dragging oncologist's office.

But once I was back on chemo, I calmed down, even though I had a bad reaction to the chemo and it only sort of worked. A year later, I'm still here plotting what I'm going to do next. (I'm currently on a clinical trial that worked better than the standard of care chemo, but the tumors are still there). I know intellectually that this is unlikely to go well, but emotionally I still have hope that if I keep trying things, it may work out. I wouldn't rate myself as unhappy.

My initial reaction was different from yours: I wouldn't describe it as fear, more like sadness. But I think it's still likely that your strong emotions will subside with some time.