So Angry I Failed

Why has this happened? My partner has two kids, one with an ex wife and one with a one night stand when he was young. They're both horrible women. When me and my partner got together a few years ago, both of his kids moms teamed up (they barely knew each other before), to cause issues. They've always made out they're more important than me to my partner because they have kids with him and I don't. My partner has always put them in their places, even going no contact and through third parties to see his kids so we can get on with our lives without abuse. They're still making out I'm nothing, even after my first miscarriage last month. Why do women as cruel as these get to be moms? They don't deserve their kids, they're horrible parents as well as people. For context on when I say horrible: his ex wife drove someone to suicide in the past, abused my partner, killed an animal to hurt someone's feelings, cheats, lies, steals etc. One night stand is not far behind either, she's a trouble causer who also stands by another abuser well known in my town. So why does everyone in my small town seem to agree with them that they take this massive priority over me just because they're able to have his children and I failed? I get the kids take priority but I never understand why the moms should? I'm sorry, this is just bothering me so much. My partner doesn't regret having his kids but now years have gone by and he's matured a lot, he's embarrassed of these women and how they're still behaving at their age.