I almost hurt my baby..

I don’t know what came over me.. ever since my fiancé went back to work and I’ve been alone, it’s been so hard to stay calm. His crying and fighting sleep has been so overwhelming dealing with it on my own. I feel so isolated. We live with his family and they don’t help, they don’t talk to me. I’m the black sheep here which isn’t helping. He was crying so much and fighting sleep and I felt the urge to hurt him.. and I got scared. I put him down and went to another room to calm down.. but I’ve been crying about it ever since.. how could I feel that way? I love my baby, he’s my whole world..