Your remarks about “…this was not dad anymore” prompts this reply. My wife has MCI (Mild Cognitive Impairment) a pre-condition sometimes to dementia. Her most dominant of the 3 symptoms is emotional impact. Anger, hostile behavior comes and goes along with ANTs (Automatic Negative Thoughts). Dark. At times she is far from her old self; as you said a “Different Person”. While imperfect, my means to contend with this is to use a pair of pseudonyms. Resulting in 3 names: her natural given name, and two nicknames. (She revolves among these personas)
One nick name has been ‘GG’. Fun, happy, and lovable. Another nick name is ‘Gertrud’: angry, hostile, dark and patently negative. If I had to be around Gertrud all the time, that would make ME useless, and I’d bail out. However, I can deal with Gertrud part time, and when it’s unbearable I get away for awhile, and re-center (as opposed to resent her 😊 😵‍💫)
And one final thought in the spiritual dimension. We are called to ‘light a candle, rather than curse the darkness’ At times that candle is lit for her, at really dark times that is fruitless and the candle is lit for others. And that lighting generates renewal in me, and gift from Him.