Don't think you fucked up, you didn't choose your step father but your mother did. If anything this will be a win in a future as your mother has shown her true colours by siding with you her daughter. I wish you all the best and I hope your stepfather finds it deep within his heart to apologize to you.

I'm a stepfather myself and I would never ever treat my wife's son anything BUT my own son. In fact he is my son regardless what anyone says. Stay strong and keep this strong and close connection with your mom and I'm sure itll all work out