BlackWolf: Every time they ask I just say that it's my own decision to make when I'm ready, and then my dad just says "well, when will you be ready?". It's really frustrating because if I try to actually talk to them about my doubts things will end badly like they did last time.
Then don't talk to them about your doubts.
Your parents want reassurances from you. I think they are fearful. Of what...who knows? It could be varied - fear that their baby won't make it to paradise...fear that they won't make it there if they aren't seen as good parents...
Regardless, what you are doing/saying now isn't working. Maybe tell them, next time they ask about you getting baptized, that you are praying about it. And tell them not to worry about you so much because you know that Jehovah loves you anyways even if you haven't been dunked. Tell them that in your heart, and in your prayers, you know that Jehovah will tell you when the time is right. They won't be able to argue with Jehovah.