My mother is old and I don't know how to forgive her. I almost made her cry today.

I saw my husband deal with this when his father had a stroke. There had been so much resentment built up over the years which couldn't just disappear. But the man in front of him needed compassion.
It is not a switch that can change but there needs to some acceptance that there will be no apology from them, no understanding reached. Just a decision to move onto the next chapter.