I would say, you might bring the focus down from the long-term to the short-term. When we look too far ahead, it's easy to feel overwhelmed even when we do not have disease. My dad has a lousy prognosis with stage 4 pancreatic cancer, and is on hospice care. There is no looking 2 years into the future, in the same way that I cannot look 200 years into the future and feel hopeful for the chances of my body to still be around. But there is hope to have a good day tomorrow. There's hope for a trip to the park on the weekend. There's hope for pain-free days and management of symptoms to enjoy what you do have. Try to turn off the high beams and offer something to which to look forward in the short term. For dad, this is visits from family. Take one day at a time, and if that's too much, one hour at a time.
Let her know that you are okay, and that you are also taking care of yourself. You might also give her the confidence that there is a plan to handle anything that comes up... that there are ways to manage symptoms and side effects.
You are both part of something bigger than anything that can happen to the cells of your body. Best wishes to both of you.