I remember back a few years ago whining about how my commander was going to make me run. I didn't want to, but that's military life. A friend of mine asked how he was going to make me... Was he going run behind me and push me the whole way? Of course I said no. He said no one can MAKE you do anything. I might choose to do something to avoid certain consequences, but no one is making me.
Your parents can get mad, ground you, yell until they're hoarse, but they can't physically stop you from being you. Is it better to be grounded with silky smooth legs or ungrounded and dressed like some dude? Only you know the answer. I imagine they're trying to do what they think is best, but they're just fallible people like everyone else. I'm sure they love you a great deal, and if they see you suffering it may, eventually, get through to them just how miserable you are being a boy.
And I don't just agree with Joanna, I agree with Jill too. A therapist would be a great idea. The worst they can do is say no.