See this answer is very relative and is case specific. The possibility of clearning/not clearing is entirely dependent on the intelligence, perseverance and capability of study and retaining information of a student. Better this criteria are, better the chances to crack an exam in short time.

But in this case I am ignoring these relative cases. I am giving you the most general and applied to all answer. Preparing for a tough exam like NEET in one year is hard but its not impossible. You have wasted your time for 11 but don't repeat it now. You have to be very specific to your study schedule and should be sincere in your studies. Remember that you have to cover up both the 11th and 12th syllabus. Now if you have coaching then you can be in a better case as most coaching provides revision module in order to coup up with the past chapter. Now if not then I would recommend you to create a schedule with equal proportion of 11th and 12th subjects. Many students focuses on 12 and wait for the last moment for 11. But this is wrong. As you are in 12 you have a scope to learn the subjects in a better manner for the 12 part and you have cut your schedule in order to get some time for your 11th preparation. Also focus on practicing problems as well as reading theories. Don't read too much theory or too much problems only. A good proportion of both these will help you to coup up with the syllabus.

For preparation tips visit the link below:

https://medicine.careers360.com/articles/how-prepare-for-neet-without-coaching

I hope my answer helps. All the very best for your future endeavors!