What should i do about this?

so here's the thing, my dad isnt a bad person i guess, i just cant help hating him, or at least have this anger towards him built inside of me over the years for reasons like him cheating on my mom, he cheated on her when twice when i was little, and i havent been able to forgive him for that, he also cheated on her while my mom was away for a year, last year, and i never told her about it or let him know that i knew, also when i was little i had this perfect image of my dad, like i used to think he didnt drink, but in the year that my mom was gone he would come home in the morning almost every day ( i never told my mom about that either) and he always smelled of alcohol, but that wasnt what made me mad, it was just mostly that he left me and my brother alone not caring if something happened to us, also hes a very irresponsible person, hes done things that have caused many problems for us, he acts like a teen ager when hes really 40, what mostly gets on my nerves is that my mom forgives everything he does, and she doesnt even realize how this affects me and my brother, like we dont even want him here with us anymore, and for me i feel like i cant really trust guys, like theyll all be the same as him, and ive tried talking to my mom once about it, but she told me that it wasnt my place to be talking about that and that she wouldnt leave my dad cuz she doesnt want to be alone for the rest of her life D: i just want a way to make her understand, or talk to her about it, idk, am i wrong?, can anyone help me?