Is missing Mass a Mortal Sin?

My mother is 82 and has dementia. I take her to Mass on Sunday when she is feeling well enough to attend. Sometimes she doesn’t want to go and feels that missing Mass is a Mortal Sin and refuses to receive Communion until she confesses to our priest. He has told her several times that she does not need to confess and to take communion as it will heal her soul. I don’t want to bother our priest again and don’t know what else to tell her.

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