Hello Priyanshu, 

First of all I would suggest you to not panic at all, and the next thing you can do is first prepare well for your 12th syllabus as its board exam and you will have to carry the marks everywhere for rest of your career, what you can do is combine your neet preparation with your 12th syllabus first, finish the 12th syllabus properly and in the mean time dedicate 2 hours everyday to revise the 11th concepts that you studied before. In this way you will end up doing well in boards as well as in neet. And one more thing every Sunday try to solve some MCQs of the chapters studied that week. That will ultimately test how much you retained. 

So try following a proper time table and clear your concepts that will eventually let you attain success. 

I hope it helps you. All the best.