Do you think they will let me change my course?

I'm a sixth form student in my final year of study, I'm currently taking two subsidiary's and doing one 90 credit in Applied Science. I wanted to do a double (Extended Diploma) in Applied Science last year but I wasn't aware the option was open, so I did a single and got a few units under my belt. Now in my second year, I needed to pick up some more periods for my timetable because I didn't do well in my A-Level I did alongside my two subsidiary's. So I picked up another subsidiary in September, and I really, really hate it and I cant get onto the uni courses I want without a National Diploma, so I'm thinking of asking (begging) my teachers to let me drop the subsidiary to pick up double Applied Science. Do you think they will let me? If they don't then I'm truly in a bad place, I feel sick with worry. I'm a hard-worker too, on course for D*.