I did this when l was fifteen, because of the way my mother was with me.
I started going out with my best friend's older brother (he was my first boyfriend), who she'd met, and l'd said l liked him, but no more, not that we were seeing each other, because l knew how she'd react.
She liked him, she actually said "He's lovely, and he's a good looking lad, he's too good for you, l wouldn't bother," and so many other derogatory comments about me.
He didn't live locally, so l could see him when l was staying with his/my friend's parents. He was lovely, older, but he treated me so well, as did his family who accepted me firstly as their daughter's friend, and also like a daughter.
I had no privacy at home. She'd go through my things, my wardrobe, l had to write my diary in code and lock his letters/cards etc in a record case, and say l'd lost the key. Even then she said 'she'd tried to get it open for me'.
I didn't tell her things because l had in the past, and had them thrown back in my face. She even did it in front of other people, including neighbours.
So frankly, OP, having read your posts, you wonder why she hasn't told you he's her boyfriend? I don't think it's hard to work out.