I do understand how alone you feel. And you've already made your parents thrilled with you for saying yes, saving your father's face as an elder. You have two choices. Both are hard. I hate that religion for making these the only choices for amazing young people like yourself.
1. Get baptized. Further succomb to all sorts of pressure to live up to your baptism. Service hours, pioneering, seeking a worthy spiritual person to marry. If you meet someone not a witness who you start falling in love with, forget it. Until you can move far away, slack off, and slowly break your parents pride and hearts. You may never be free, if you move somewhere where someone knows someone from your area. The witness world is a small one. You will always be worried.
2. Tell your parents you are so sorry, but you wanted to make them happy and say yes you would get baptized this summer, but the truth is you are NOTready. I really like DOC's suggestions (except the getting drunk one :)) about telling your parents your 'doubts.' Heck yes they will be disappointed, but why let this religion control your whole life, forever? Share your doubts head on, without implicating or admitting any apostate sources. Make them have to answer the tough questions.
I think you must be referring to an incident where you got in trouble and it caused a big ruckus and confrontation and it left s very bad taste in your mouth and unwillingness to cause any trouble. But you can speak to them not as a child but as a student, no shouting involved maybe. And if your father does lose it, ask him if that is what he thinks is correct..shouting, coercing a baptism into someone. Through everything be sympathetic to their viewpoint and their own experience. But it is NOT too late.