I am so sorry you are going through this, it is completely shit. We didn't have a lot of time to prepare as like @narcASD my Mum was symptomless and found out she had stage 4 cancer from a routine mammogram. 10 weeks was all we got from seemingly healthy to gone. The last few weeks were torture.
What I take comfort in is that I knew her so well I can pretty much guess her side of a conversation. I still "speak" to her, sometimes aloud, sometimes in my head and I can imagine her replying, I can hear her voice. I know what she would say, what she would advise and as much as I wanted to fall apart I knew that she would have been the one to help me back up off the floor and crack on because life is for living.
If you can, record your Mum talking because what they sound like is often the hardest thing to play back in your mind. My Mum died 15 years ago. In all honesty you just live with the pain of it and it will still cut you to the core when you least expect it. Mother's Day is bloody awful especially as everywhere you go it is in your face.
But you also remember all the good things, her knowing your children, being in your life. You basically try to drown the fact that she isn't here with brilliant vibrant memories of her, her funny phrases, the way she laughed, the books she read to your children, the voices she put on for it, how beautifully she sang (if she did, mine did) we play the card games she taught us from childhood, we talk about her.