I can truly sympathize with what you are going through. I am an only child as well. My step father was verbally absuive toward me and physically and verbally abusive to my mother. It was not an ideal way to grow up. I found that focusing on studies and after school extra curricular activites was a good way to focus my energies. FOr the most part I tried to stay out of sight of my step father that his impacts on me were minimal. I got a job at 18 and moved out of the house shortly thereafter. He died about 15 years ago, so thankfully he is no longer even a consideration in my life. I paid for my college education using a federal pell grant, and after that a tuition reimbursement program my work had. Now I have a great job, and own my home. I think any parent would be quite proud of that.
I wish you luck. In the meantime, this is a great place to hang out with father figures and bother figures.