We keep saying sometimes, acha kar nahi sakte toh kam se kam bura mat socho (acha socho). The same needs to be applied for ourselves.
How much you study, how much time it takes, how much you understand, how well you have done in the past etc takes the 2nd place. First is how you think. And all that I can say is you should always think you will do well. Most of the times, if we are not able to study, or if we see others studying more we start feeling we wont do well and loose confidence. The key is to keep the thought of doing well intact. Even on the day of exam, if you feel you havent studied well, its fine, but that should not make you think that you will fail. Even in that case you stick to your thought of doing well....(by not studying well you havent done good to yourself, atleast if you cant do good to yourself, think good...it doesnt cost anything to think good always).
I think it makes sense to appear for both the groups...you may focus on one group only if you feel two groups would be too much to study in the remaining time. However, even in that case, appear for both groups, you never know how lucky you can be, rather how lucky your positive thoughts can make you!