The very best thing that you can do for her right now, is to show her that you still love her and that you are willing to do whatever it takes to make her feel better. Right now, if she is that sick she will need your help and won't be happy being the way that she is. She is relying on you as a friend, as a mother, as a carer, and most importantly someone who really needs you right now and who loves you to the ends of the earth. If you truly love her, and there is nothing that you can do, think about what is best for her and show your love that way. She will definitely appreciate it, and will realise that you are repaying her the favour that she always gave you. Stand by her side like she stood by yours, and your love and friendship will be unwavering, unbreakable and everlasting, no matter where she goes. Give her the best treatment, and make her last days the happiest that you can possibly give her, and if she is in pain, then let her move on easily than delaying it.
If she is ready to move on, then she will be not be lost but will be experiencing another kind of evolution, and will be getting ready to explore the next world, and the closest and most peaceful, hidden dimensions that are with us everyday, but which we ourselves are not evolved enough yet to see. Therefore, she will always be there around you, happy, peaceful and watching.
Some people believe that there is no such thing as death, merely evolution, and that in the next dimension which we then gain access to, there are houses and people and civilisations, and organisations that look out for us. She will not be forgotten, there will be people there to take her in, feed her, care for her and look after her. She will gain a new family, and be able to help them and stand by their side and the side of their children just as she has done for you. She plays an important role in society, and in the spiritual world, and you should be proud of just how much she has done all by herself, and what exciting things are going to come for her yet again.
Then when it is your turn, you can go and find her and meet the new family/s who have been taking care of her, share your experiences, and become an even bigger family. It is something to be very proud of, and something exciting to look forward to. So instead of thinking of loss and sadness, think of pride and the ability to make important decisions, and become even more of a man than you obviously are at the moment. Think of growth and exploration, and connecting with mother nature, and this wonderful, beautiful secret access to this magical place that very few know about. This way it feels more like your own child growing up, going to school, finding relationships, going to Uni, getting married and emmigrating, and you having empty nest syndrome, but able to be proud of who and what your child is.
But for now, there are always bereavement services to help with these times, so best of luck and my thoughts are with you.