My 19 year old girlfriend was recently diagnosed with breast cancer. I don't know what to do

This is really hard for you too! 

Right now your girlfriend is swamped with fear and sadness. Just keep letting her know you are there, that you care, even if she won't see you yet. Is she getting positive help elsewhere? From her family? She needs advice about living, and help to focus on what she can do, at a time when she will feel very powerless. 

You are right to try to support her, even if she feels she can't ask this. Make sure you get some support too. These are very hard times for everyone.