I have already failed in those exams, mostly because my preparation was not enough

准备不充分考试没考好

The part of it that worried me the most was...

令我最担心的部分是...

The best way that I could

尽力

I chose to dedicate 6 to 7 hours each day

每天拿出6-7个小时

Put the accent on

重点加强

I declined several invitations and dates for that

为了学习推了约会