Don't let your mom tell you shit. From the looks of it, you are the adult and she is the child in this situation. You did nothing wrong, and exploring your sexuality is normal and healthy behavior for a teenager. You even have the sense to do with an underlying motivation (working through some bad experiences), and isn't just monke horny motivated. Her behaving like this is more than likely to add to the negative experience than to solve it, and no parent should treat their children like this.

It sucks to say but there also isn't a whole lot you can do. Like your mom says you are still young, and mostly still dependant on your parent(s). The only advice I could give you is try to have an open and calm conversation, where you clearly relay your motivation and opinion on the matter. Getting angry or upset, no matter how right you are or feel, will only reinforce their idea that you are "just a kid", whereas calmy openly discussing it will show some maturity and thought about the whole thing.

Edit: a thought came up to me. Maybe a smart move would be to tell your granny that a SERIOUS boundary has been crossed by your mother, and concrete actions have to be taken. I'm not talking about punishment or revenge, but this dynamic between you and your mother has to change. It's not healthy for you to have to hide things for your mother, and it's not healthy for her to completely stomp over all her kids wishes and boundaries. Maybe suggesting a talk with a mediator or psychologist with you and your mom will show both her and her granny that you have reached a limit, and something HAS to change.