I just wish I could tell you I have the magic bullet right here in my pocket, OP. 😢
And although it's my SIL who has walked this walk, her husband is, of course, my brother. So I had a birdseye view of how bloody hard it was for her to stay strong, optimistic, and supportive when she herself was just disintegrating with fear and drowning in sorrow (and of course, my brother was well aware of the fact that this was so damn hard on his beloved wife). God, it's the devil's work, cancer. It tests couples and families beyond anything anyone should ever have to endure.
You brave, brave soul. Do you have anyone you can just fall apart with? I know that during the 4 years of my brother's illness (pancreatic cancer- never a good prognosis), my SIL really never got to a place where she was beyond welling up. The tears never stopped, I am so sorry to say. But what she did do was accept who she became as a result of her husband's cancer. She stepped over a life-changing threshold and she learned to accommodate all that this step came with... and took away.
You are in grief. It is a bereavement. Even when people survive cancer- and so very many do- it is a bereavement, a mourning for the life that once was, and a reluctant acceptance of the life that has become something else. And you learn to live in that new space, with tears, with anger, with grace, with dignity, with hope, with despair... so very many strange bedfellows. Cancer really is life on life's terms. And if there is one thing cancer, a taker of so many things, actually gives, it is the power of love (so painfully corny, I know, but it is the truth. Every one of my 7 aunts and uncles has had cancer- add my brother and mother to the mix and I must declare that the love that saw them and all of us through the worst of times is powerful beyond description... at the saddest of times, love is everything and it is your strength now, OP).
I've offered no advice- just words. But they are loving words and I hope, with the whole of my heart, that your DH will live a long life... one as a cancer survivor, a soaring phoenix in flight. 💐