I understand, Rich. I think.
Cancer has taken all the women from your life. Your mother - gone. Your sister - going. Your partner - stage 3 isn't good, she's near the door.
It's ineffably sad. I am so very sorry this is happening to you.
I imagine you must want, more than anything ever, to be able to help your partner and even make her a little better. And you must be feeling, quite desperately, the loss of reciprocation from the women you love.
They cannot reciprocate. This is awful for you. Every day, you love your partner and she's too locked in her own grief to love you back.
She must be so full of anger at her disease, and so full of determination - because it takes courage to go through those brutal treatments. I think perhaps the tight face you see is the face of courage. To fight pain, grief and fear at her level takes everything. Nothing left to give for your feelings. But, you need to believe she hasn't left you yet; you give love and it's like no-one's there.
She hasn't left you yet! I would think she knows you love her, but has no more resources to love you back just now. As a small practical suggestion, have you thought of bringing her a small gift each day? Just something tiny, that you know she'll appreciate. A chocolate bar, if she still likes chocolate, some fresh fruit if she can taste it, a couple of her favourite flowers, a scarf, a lipstick, a magazine, a DVD ... Perhaps some of these things won't bring her pleasure if she's in pain or zonked out, and perhaps some will. She'll know you're thinking of her.
This is all 'give' from you, and it's so unfair. It just is. I think you probably need a release of your own. Separately, I also think you should talk to your doctor AND hers (if different) about the huge emotional strains at home. There are organisations and programmes to help both the cancer victim and their partners: access them, please, for all your family's sake.
Wishing you all a peaceful & healthy outcome.