Wow, it sucks to be in that position :-( It breaks my heart a bit...
However, big kudos to you for talking to your mom and standing up for yourself! Well done! Learning to make your own decisions, and learning to say no are most important when growing up. You took some steps there!
I know you probably just need a hearing ear on here, but hey, I am guy so I only try to offer solutions ;-)
Is there a nice older and wiser sister you're a bit close with, whom you can tell the 'I feel pressured' part of the story?
And I'm wondering where the whole dedication to Jehovah in prayer is in this story. Do your parents think you've already dedicated your life to Jehovah? If not, you can try to reason with them:
"Baptism comes after dedication. It's a very personal decision. Right now I feel pressured into hurrying to make the most important decision of my life because convention is in 6 weeks and grandma is coming. I don't feel that's fair. I want my dedication to Jehovah to be something special, and right now it feels like its just another appointment on your schedule to keep. I don't want it to be like that. I will get baptised, but when I am ready."