I am so sorry about your mom. I just lost someone very important in my life. It started with breast cancer too, then spread to her brain. All along we thought she would beat it, regardless of what we all read about, and kept a positive attitude. In the end, she did not make it and had a long, painful death. It was one month today that she passed. The hardest part for me, is similar to you. In the mornings, I wake up and realize it wasn't a bad dream, yet it is true and she is not here anymore. No more phone calls to say hello, no more funny stories to share, no more memories to make, nothing but pictures to look at and reminice in my mind. She was 35.
Your mom was young to die. She still had many fun years ahead, especially being a grandma too. It must have been awful to think all this time your mom was going to conquer it, yet to realize in the end, she could not anymore. It made me annoyed when people said, "At least she is not suffereing anymore." It comes to a point when that just isn't good enough to hear anymore. However, at my friend's funeral, the priest said something that I hold onto at night and in the mornings when I cry. He said, "We cannot look at the quantity of years she had, yet focus on the quality of those years." I hold onto that. I stop and think, she loved, she was loved back, all of her dreams came true, she just did not live long enough to enjoy them to the maximum.
Hopefully what the priest said will help you get through the many tears that you will be shedding and again, I am so sorry you lost your mom.