A cancer diagnosis is one of the scariest things a person and their loved ones can experience; it’s totally normal that you have a range of emotions. I suspect the hurt and betrayal you say you feel by your mother’s behavior is actually fear, uncertainty, anxiety, and helplessness re-branded in your mind as emotions that best reflect the kind of victimization we feel during traumatic experiences.
In doing this, you make your mother’s cancer about you – your hurt and betrayal, when in fact it’s your mother who is suffering the most, and any discussion you have with her should center on that one fact instead of on your own pain. She is already carrying enough burden, so don’t add to it; remember that your desire in being kept in the loop is to provide support, so you want to show her that you can be supportive and not make her illness about you and your needs.
Here’s a simple script you can use and tweak as it makes sense for your situation and relationship with your mother:
“Mom, I am concerned about your health and well-being, and I want to be there for you as part of your support system as you fight cancer. I know you’re my mom and it’s usually you supporting me, but I’m a grown-up now, I can handle adult responsibilities like caring for loved ones who are ill, and it would mean so much to me if you could entrust me, like you do with Dad and Aunt and Sister, with details of your tests and treatment. I love you, and a really important way to return the love you’ve given me all my life is to support you through this challenging time. Please let me do that.”
I’d also suggest reaching out to your dad (or maybe your aunt or your sister), if you don’t make headway with your mother, and let them know how important it is to you that you be kept in the loop. But again, center your mother in these discussions and your desire to support her and the rest of the family, and not your feeling hurt and betrayed. Centering your own pain here does the opposite of what you want, which is to convey your ability to give (emotional support, love, maybe even logistical help) rather than take (emotional support from others to tend to your hurt feelings).