I am surprised and disappointed that no one has replied to you before with compassion and empathy.
Having lived through a brain tumour and surgery myself I acknowledge that my behaviour was affected and I only processed the odds of a shortened life without facing certain death. I had to acknowledge to myself and my family that my behaviour was hard on them. I imagine the cancer has affected your partner's behaviour just as mine was affected and I am CERTAIN that cancer and uncertainty and fear have affected my husband's emotional regulation...
BUT you are human too and I affirm and applaud your honesty, your willingness to consider what is right and compassionate and most helpful. I know the world needs more givers like you.
I am sure you made your mistakes in the relationship but you still want to give what you can and setting boundaries is best for everyone involved.
I have experienced already how easily people show compassion to the cancer sufferer but how much less support there is for their carers.
I cheer you on with doing the best you can with what you know, for forgiving yourself and your partner whatever comes next. it is OK to be human.
I can offer love and prayers
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