Tired of coming second!!

My SO had a child with another girl.. Which I love his child.. But the mother on the other hand I can't stand.. She is a piece of shit doesn't really take care of her children.(she has three children that aren't my SO the youngest child's father lives with her) If she needs something my SO is the first she calls to ask for help.. Today we get woke up to her needing money to pay the Duke bill. Well we have bills too.. His like yeah give me a few days to get it I'll ask my parents for it.. Well I hear this and shit hits the fan. I'm like why the hell do you need to pay her bills and stuff if her drug addict dude would get off the drugs and work they would have. She gets child support but yet we still by everything for his child. His baby mom can't even put matching shoes on his child he comes in clothes three times his size and looking a hot mess.. I'm like wow my child never leaves the house like this.. Than when I say my peace I'm wrong. I don't see where I'm wrong.. I'm too the point that if he pays it I'm done.. I will pack mine and my child's things and leave.. She does this all the time and he always bails her out and I'm tired of it.. I feel like the things we need always get pushed a side for her.. And that I'm not important.. Am I wrong?? Would you ladies be mad to??