Im 13 and I play with myself all the time!! It feels good and it's relaxing!! I've been doing it for a while and it never is boring. But whenever it's bought up around people it's like the worst thing you can do to yourself!! Why?? How can something that feels that good be a bad thing for you?? Is it just me???
I suggest you read a bunch of the other posts on here about masturbation. You are certainly NOT the only girl who masturbates and your question about why others pretend they do not is as old as the ages.
When I was your age (60 years ago) all the guys knew the other guys did and all the girls said they didn't (they were lying). It looks like things have not changed much.
Also it probably one of the best things you do for yourself, there is nothing bad about it.