Very to sorry to hear about your loss.
I would say - just for the OP’s sake, as she’s clearly feeling guilt - that this poster’s mother acknowledged her cruelty. When a mother does not, it’s very difficult to forgive.
Living with someone who berates BERATES your father constantly in front of you, or turns every conversation into a negative, is incredibly draining for a child. It pulls you down, teaches you no empathy, messes with your understanding of the world and impacts your relationship with your own children.
I am 99.9% sure (seeing as the op is here after having snapped and is feeling shitty and conflicted about it) that this isn’t a case of ‘unrealistic expectations’ or ‘impossibly high standards’ on the OP’s part. She will likely have lived an entire life forgiving…being generous to her mother. Allowing, and probably even enabling, her bad behaviour. Putting up with cruel comments or dealing with the weight of judgement, or tiptoeing round her mother.
Also, when someone stands up to their abusive father, snapping as a teen for example, and jumping to the defence of their mother, everyone is like you did the right thing…he was a prick…he was abusive. So why when it’s a mother doing the abusive behaviour should this woman stand there watching her father be belittled over a bloody cereal box without saying a word?
Not only this, but it is quite possible the OP’s father just does what he can to have an easy life, to keep the mother placated, and therefore doesn’t defend the OP or stand up to the mother properly. That causes a whole host of other issues. (Obviously I could be wrong in this case because I know only the post’s info but OP worth looking up covert narcissism as rings bells)