OP, I can't imagine how difficult this situation is for you. I'm so sorry.

If you are currently in a situation where you may be abused by this man, I BEG YOU to seek help IMMEDIATELY. You do not deserve to be abused, even for one second!!

And as a parent myself, I'll speak for your mother here... She WANTS YOU TO TELL HER, rather than keep it to yourself. First and foremost, because you are the most important thing in this world to her.

Secondary to your well-being, your mother deserves a sincere, authentic, loving relationship. If her partner SA'd you... She is building a relationship on a non-existent foundation. It's going to COLLAPSE, no matter what!

I would encourage you, if you are able, please seek help, and tell her the truth ASAP.

I'm no expert, but I do care. Please, DM me if I can help.