My girlfriend is terminally ill at age 19 i want to marry her what should i do?

My girlfriend has a very serious type of cancer of the blood called Chronic lymphocytic leukemia, which requires chemo and heavy treatment. Doctors are saying they may try a bone marrow transplant but i read about it online and generally there is a very low survival rate, the doctors have not told us anything about life expectancy etc. I love her so much i want to cry and sometimes i do, we have been together for 3 years. She always told me about her dream of getting married and having a honeymoon in spain and just living a normal life. I am only 20 but this is the woman i want to marry, i dont want anyone else ever. She feels very self conscious now she has lost her hair and even tho i tell her i thinks shes pretty she gets sad because she thinks im lying (but im not). I told her parents my plan to propose and have their support, im just worried how a terminally ill person would respond...would it make her more sad because i dont wnat her to be sad : (