Both of my parents died of cancer when I was a teenager. It's tough. They died at home; we cared for them the best we could. My sister was and still is an RN so we provided them with the best care possible. This was in the early 1970's.
The point I'm trying to make is this. She is dying, but don't treat her like she is anything less than a whole person. Every day from now on is a gift. Treasure the time you have left with her. Keep her comfortable and afloat in love. Talk about good things and good times. We tend to treat sick people as if they are separate from us becuase of all the pills, bandages, smells, etc.
Though she has cancer, don't just see the cancer, see the person. Obviously plans have to be made. If she hasn't already arranged her affairs, she will need help. Y'all will know when this topic will need to be brought up.
Being a care-giver is very hard on a person. I took care of my mom by myself at times. I was a sr. in HS. It can leave you with emotional scars. Ask for help.
Hospice is a great system that provides in-home care. My sis is a hospice administrator. She would tell you that if you get on with a hospice, to be choosy with who you allow in your home. Some hospice workers are great and some are crummy. Be picky.
The mourning has already started. Accept that. Death is part of life. You will have times that you will want to scream and curse God. That's okay because God understands. Do what you can to avoid dramatic emotional events in front of your mom, but crying is okay.
Some well-meaning people will tell you that if you just had enough faith, if you pray the right words, if you really were close to God, that your mom would be healed immediately. Don't buy into that. Jesus healed the sick so that the people around Him would believe what He was saying. The healing that God wants us to have is in our hearts.
Love yourself and be gentle toward yourself. Don't feel that you can't laugh and smile at these times. You won't come out of this unscathed but you don't have to be broken forever either. This will change you. Try to make it change you into a person with a gentler heart.
I pray God's richest and heart-healingest blessing on you, your Mom, and your friends and family. Someday we will be restored and all together again.