How do i deal with my dad's girlfriend?

my parents are still legally married but for about 5 years they've been separated. It was really hard and still is. My dad's girlfriend is the girl he cheated on my mom with. I don't see my dad very much but when i do, i don't know how to act about his girlfriend. Right now (and when im just thinking about it) i seriously just want to punch or spit right in her face because she ruined my life. My dad owns a business and she's only after his money. Now my mom and us kids have to cut back our lifestyle while he supports this tramp. But then i think about it again and i guess shes human? idk. i'm a really nice person and know i will never like her but i hide it because i've been raised to respect my elders. Im not sure someone like this deserves respect though. She's changed my mom from a determined, strong woman into an unmotivated, weak one and tore our family apart. I know its a lot..thoughts?