OP- I am in a similar situation. Parents divorced at 13, I was forced to grow up fast as my mother was depressed and basically absent and my dad was clueless on how to raise kids. Dad remarried immediately (I still think he & second wife planned it all out) and moved on to his new "perfect" family, leaving me to take care of my mother. I didn't realize how bad it was until a few years ago. My mom was on all kinds of anti-depressants and just kind of empty during my teenage years. I really raised myself when I think about it.
This is awful, but now that I'm older and having to take care of my mother, I realize I would have divorced her too if I were my dad. Luckily he got to move on, I get to take care of her now. She mentioned moving in with me in a couple of years and I had a panic attack. She hasn't dated, lives in the same sleepy town, has let her divorce define her, and it is just sad to watch. I try encouraging her and I get scolded for being "critical".
Therapy helps. So does introducing your stepmother as "my father's second wife"