Feeling Indifferent

I'm trying to understand myself better and I hope that there are mothers who can guide me through this plight. Eversince I gave birth to my second child, I experienced PP rage and an absolute disconnect towards my firstborn. It seems like an out of body experience. Each time I scold her or hurt her, It's as if I cannot control myself. I just lash out but then I will come to my senses and apologise to her. I don't think I'm tired because my second has been quite a breeze since 2 months.. often I have to wake her up for feedings or else she just sucks her thumb and MYOB. Now that my secondborn is 4 months the my PP rage seems to have died down BUT yet I still feel the disconnect towards my firstborn. I don't feel genuine it feels so awful that the feeling I once doted her and could cuddle her with love, I don't feel that anymore. It sucks. What should I do?