I Refused to Cover for My Coworker While She Took Her Miscarriage Leave

I work in a small five-person team. Recently, my coworker Anna had a tragic miscarriage at six months and took two weeks off. She was leading a project, and since I’m the only one trained in her specialty, my manager asked me to cover.
I refused. I had already covered for someone the month before and was exhausted. The team is now overwhelmed, the project is failing, and they blame me.
What they don’t know is that a year ago, when my mom was hospitalized, Anna covered for me, missed a deadline, and then accused me of lying about my mom being sick to escape responsibility. She pretended nothing happened afterward.
Because of that betrayal, I couldn’t bring myself to help her this time. The project may fail, but I won’t sacrifice myself again for someone who once tried to ruin me.