How do you cope with a terminal diagnosis?

My mother battled ovarian cancer back during the Covid years and was declared metastatic(tumor free) but as far as I and my younger brother were concerned it was basically a win against cancer. She stopped having to do chemo, she grew her hair back, and she did regular semi-monthly checkups with her oncologist to make sure the tumors didn't come back. Life returned to some sense of normalcy for us.
Then in January of this year, on a routine screening we got the worst possible news.
Stage 4 terminal cancer, on the back of her skull, her pelvis, both arms, her left femur, and along her spine.
Several of those places the doctors said would be impossible to operate on even if they did respond to chemo.
They told us with chemo she had maybe a year to live, if not less.
My brother's 21st birthday is the end of January, my 30th is late February.
There's a real chance she never sees them happen. She'll likely never see met get married, or buy my first house, or have kids.
And it burns a hole in me. I find myself thinking of all the things my brother and I will get up to as adults that we'll never get to share with her. I remember too much of the ways I could have been a better son, or how unfair it is that she's gotta go through this after working 2 to 3 jobs to raise two boys to manhood in the first place.
The grief feels like a vice on my soul. There's not a day where the thought of her being gone doesn't lower the quality of my day. No victory really feels momentous anymore. Every day at work, vacation, or other duty feels like time I could be spending with my mom before she's gone.
But what can I do? What is there to say to her, when she's the one actually dying? How do I talk about this with my brother?
The weakest part of me just wants to run away.
I put on my strongest smile for her whenever I see her, but I have no words to offer her or my brother, or words to cope with what I feel.
What is there to do?

-Elijah