A/N: Having gone through chemotherapy myself, this is my perspective on why Jane keeps picking up Mjolnir.
The people who know that Jane Foster is Thor, who know that every time she picks up the hammer her chemotherapy stops working, those people assume she keeps picking it up for selfless reasons. She is forsaking her own health to help others. She is doing it because Thor Odinson no longer can. She is doing it because the universe needs a Thor to protect it. And that's true…to a point.
Picking up the hammer is also selfish. And every time she does it, it gets harder to put it back down. She is strong as Thor. She isn't nauseous or weak. She has hair and no dark circles under her eyes. Food tastes like food. Water tastes like water.
She can freaking fly.
As Thor, people aren't telling her how good she looks. (They never add "considering" when they say it, but it's always there.)
Thor is ready to take on Roxxon, Malaketh, Loki. Jane just wants to crawl into bed and sleep.
Being Thor is selfish. It makes her feel normal, strong. It lets her world be defined by something other than cancer. She likes that feeling. No. She loves that feeling.
Now her friends are taking that away. She understands why. If she were in their position, she'd do the same. It's killing her after all. It's not allowing the chemicals to work, the chemicals that feel more dangerous than the cancer.
"Thor is needed," she tells them.
"So is Jane."
She knows their concern comes from a place of love. It doesn't make it easier though. Being stuck in a hospital bed cannot compare to being in the sky. She wants to call to Mjolnir, but she promises not to.
It's so much easier to fight a minotaur than swiftly dividing cells.