@Cassie63A Do-Over, Charlie Foxtrot's A New Past. The girl is Wendy, but the MC refers to her as Abby Normal. She appears in Ch.3, and checks out in Ch.4, having a tumour in her chest removed.
"I'm going to be in Bloomington, in the hospital," she managed to get out before I felt the tears drip on me.
I was at a loss. I stroked her back and hair and waited for her to calm down. "It's alright, Wendy. It will be alright," I kept repeating.
Eventually, she regained her composure. "I wanted to tell you tonight, since I have to start talking to teachers on Monday. I wanted you to hear it from me instead of your mother. It's the reason I can't let you be my boyfriend, even though I've thought of you that way for the past couple of weeks. This is serious."
ETA: Eventually I got to the other identically-named question posting, where this had already been answered. Oops.