Dw Kon, you are not alone. I hate humans.
Jokes aside, I like the depiction of Mari's situation. She has to 'parent' her mother. As in, 'protect' her, make sure to keep her 'emotionally stable' by avoiding triggers etc. I also like how she copes with the situation, 'this is fine', 'all humans have their weaknesses', 'this is normal' etc.
Realistically, this situation at least has an eventual end once Mari is an adult and manages to move out/cut her mom off. (easier said than done, ik)
Someone mentioned 'why is the mom acting like that when she is the guilty one? The one that cheated'. A lot of ppl that do bad things do not accept the guilt. 'everyone does it', 'this and that person's actions are so much worse', 'it's my husband's fault for neglecting me/not showing enough affection', etc. It is easier to run away from your guilt/redirect the guilt than accept it.
You can also see how doing so makes the mom's life in some ways hard as well. See how she blames everything and everyone else while throwing her temper tantrum. When it's not your fault you also cannot take action to remedy the situation cos you cannot change other ppl's behaviors/actions.
Also yay for Kon's development, taking action to help her friend.