I slammed the front door behind me as I dashed down my driveway. I didn't know where I was headed, and I didn't care; I just wanted to get away.
Tears distorted my vision and anger pulsed through every vein in my body. She had hurt me; the one person I cared most about, my mother, had let me down. My life was changing drastically and I
was clinging to any source of stability I could grasp. But there was nothingfelt I had no source of stability to cling to. I walked fiercely over the autumn leaves that blazed red and orange against the pavement, like the blistering emotions burning inside of me. I sat down when I reached a cleared field at the top of my street, out of breath.
I began reminiscing on the past couple of months. How did I get to this point? I wondered.
Following my parents' recent divorce, I discovered that my mother had been having an affair with my new stepdad.
This sounds strange. I assume you mean you discovered she had been having an affair with him *before* the divorce, or else it wouldn't be infidelity, but the time indicators you mention don't actually make this clear.
"
Along with taking up infidelity, She had also
picked up his habit of drinking alcohol excessively.begun drinking heavily. "
"I have continued to do well in school
since then .
following this time period in my life. "
You open with a narrative frame, and it seems odd to me that you don't come back to it in the conclusion at all. I don't know if other readers will feel the same way, though.