How can I deal with my stepdad sexually abusing me without ruining my mothers life?
You can't. You must put a stop to it by reporting it. There will be great hardship, anger, and grief. Those will pass in time.I'm sure your mother should be more concerned with your welfare than her life being ruined.
Tell your mom. If she's willing to let it go without doing anything, why does her feelings matter? Then call the police and file charges against your stepdad.
You should be your mother's top priority! If she is allowing someone to physically, sexually or emotionally abuse you, she is committing a crime and should also be reported!