Cancer is not uncommon in my family, but I was still totally unprepared for my own diagnosis.

In June 2015, I was a healthy 43-year-old mom and first-grade teacher who never got sick. Then I noticed a pain in my left arm while I was packing up my classroom. It had been bothering me for a couple of weeks, but I hadn’t given it much thought. I nursed it for another month before finally going to the doctor. She said I’d probably pulled...