Fear of Cancer harming relationship

Hi everyone, my name is Eliza, and I need help dealing with a fear of cancer. I am reluctant to say I am cancerphobic, because I have only been struggling for a few weeks now, but I am terrified and constantly crying just the same. My boyfriend, who I am so in love with, had cancer as a child. Because of this, he has an increased risk for future cancer, though he has been cancer-free for 16 years. I'm not sure why, but this increased risk is hitting me HARD now, though we first talked about it 5 months ago. I keep picturing a future that is nightmarish to me-where, in marriage, he gets cancer again and dies, leaving me alone and in poverty, maybe with children. This image is weighing heavily on me and tainting our relationship, because I feel like saying yes to him means saying yes to that miserable future. I am even considering breaking up with him because I don't want that future, but I know that's dumb because it'd be impossible to find someone with NO health risks of any sort. And that's the only reason I would leave. I'm terrified of losing him and saying yes to that nightmare. I know it's ridiculous to be crying constantly about a risk of cancer, since it seems like everyone has risks of diseased, but...I don't know what to do, how to deal with it. I'm panicking. Please help.