Hello Lynn, I know only too well how very hard it is for you at this moment. You don't think you can handle the scene that may happen if you talk frankly to your parents about your mother's illness. You will cry, and they may cry, and you will be embarrassed. It will be emotionally upsetting to all three of you.
It is so much easier to avoid the subject, to avoid seeing the illness, to pretend that it's not so bad.
I was 20 when I was in the same situation, and I will never forgive myself for my cowardice. It weighs on me heavily. My mother had cancer, but it makes the same difference - I did what you are doing, and lost my chance to help.
Be brave, wade in with both feet and tell your mother how much you love her, and ask her questions about her illness. Listen and emphasize and ask how you best can help. And cry together.
I have Parkinson's and I talk to my adult children about it. At times they retreat into denial and don't like to hear about it, but most often they listen and ask questions, and it is a great help to me when I see that they care.