What have I done to my life?

Two years ago, I was a 28 year old bachelor living in my own house with a steady job and no debt aside from my mortgage. I started dating a girl with two children (daughter 8 and son 6). I ended up getting her pregnant out of shear stupidity. She pressured me to have her and her children move in with me and I agreed. It all happened so quick and unexpectedly, I didn't have time to think. Plus I didn't want to be that guy that gets a girl pregnant and doesn't have anything to do with the child. The day she moved in, all hell started breaking loose. Gone were the days where I could come home to quiet after a long day at work. Her kids would constantly fight and we had different ideas on parenting. I found it hard to discipline her kids since they weren't mine but slowly began to play the "Dad" role. They obviously started resenting me and she would always tell me I was wrong to try to discipline them so I stopped, hoping it would resolve itself. We fought constantly. When it was time for her to have the baby, I was vehemently against giving him too many vaccines. She just rolled her eyes and said "you're not a doctor, what do you know?". Fast forward to today, he's been in the ER no less than 10 times for all sorts of ailments (asthma being the most extreme). Most recently however, he's been diagnosed with autism. I'm devastated. Broke. In debt. In danger of losing my job. In a crammed house. Girlfriend with no ambition to work a full time job. All because of that one careless night. I feel so drained. The only thing that gives me any motivation is my son but I feel I'll be fighting a constant battle now.