Why can't I do it?

If you don't feel nervous about masturbation or sex, it could be that you have a very thick hymen. Lots of girls have this and it makes sex and masturbation uncomfortable or painful. Right now, there's probably not a whole lot you can do except to do things slowly. This may be kind of explicit, but you could masturbate with objects that are thinner than your fingers and gradually use slightly larger items until it's not painful to use them. Normally it will break from sex, masturbation, or even things like playing sports or tampon use. But, if it's too thick, it may remain intact.

If this is still a problem when you get older, you can undergo some minor surgery which involves having your hymen clipped. The sound of that probably makes your skin crawl, but it can be helpful. I would say this is an option when you're an adult because I have a feeling your parents wouldn't be too pleased if you asked them if you could have surgery so you could have sex.

As a final note, don't ever feel like you have to have sex. Even if all your girl friends are doing it and they tell you you're not cool for not doing it, pay them no heed - you go for that when you're ready, and not when everyone else is ready for you.