my mans ex-wife of 12 years

I am engaged to a 32 year old man with 3 kids and I have 1 daughter that he adopted. His ex wife (also 32) hates me. I have only tried to be friends with her. Yeah right, right?? She constantly harasses and torments us and tells me he downgraded and that there is no amount of makeup or wigs that can make me look better than her. She even went as far as to call my biracial daughter a "nigger" & "bastard trash." I want to stand my ground but sometimes it's hard to not break down and to be the bigger person when I just want to stomp her ass into the ground. They were married for 12 years and she left to be with "true love." A 65 year old man. I know I am not less than her but it's still hard. Sometimes I just want to leave but these 3 kids have no one else to take care of them while he works 12 hours a day. I'm the one they see 80% of the time and I am getting attached after a year together.... #stickysituation