Since My Mum Started Chemo, My Mum Has Become Unkind

I feel really bad feeling this way towards my mum, but I feel I have lost my best friend. Ever since my mum started chemo for stage 4 cancer, she has come impatient, erratic, unreasonable and sometimes nasty. I feel like I'm already mourning the loss of my 58 year mum, whos been replaced by another lady. My mum is not the same woman anymore and I feel shes aged dramatically, perhaps not physically but certainly mentally. 

Me and family are all trying not to react to her behaviour and we are all trying to show compassion for her situation by being patient. I know she must feel such deep sadness and anger at her situation, but she can be so snappy and hurtful; it can be so hard to deal with when we are all trying to be nice to her.

She has become deeply religious which has given her lots of hope, which is great but unfortunately she has become quite neurotic with her beliefs. She believes that the devil caused her cancer and watches life after death videos of people going to hell, which i think is really unhealthy for her, so Ive tried encouraging her to watch happy videos instead.

It makes me feel sad expressing negative emotions towards my mum, as she has been the most supportive and devoted mum, who has helped me in times when I needed her the most and I love her dearly, but now I feel in this moment in time she has changed so much. Shes currently being nasty to me about spending time with some.family members with my 18 month old son, when these family members live abroad and have travelled a long way to see me and my son. They dont get to see us every day but i see my mum almost all the time. My mum is very jealous of me spending time with them and she is now punishing me for this by sending hurtful messages even when I tell her i love her she tells me it means nothing. 

Overall, my mum is doing very well living with cancer and the doctors are very impressed with how stable she is with the disease. However, I dont like the lady she has become.  

I feel like cancer has already stolen my mum from me. 

Can anyone relate?