Hello Sahil,

It is not a big problem that you were not able to score good in your 12th grade.

As you've mentioned that you'll be reappearing for the improvement examination and are preparing for NEET.

Just keep one thing in mind in that you needn't worry about the competitive exam and prepare for the 12th reappearing exam jsut from the scratch and then while preparing for that just focus slightly on somewhat extra knowledge regarding to neet syllabus.

You will not face any trouble and it will be a cheesewalk for you to crack'em both :)...