Hello Dhiru, 

Look it's a misconception that 75% is necessary for getting into any college. You only need 60% in physics, chemistry and maths combined to get into any private or government college. However for getting into IIT's you need to score minimum 20% of respective board topper. Although you can apply for JEE mains and you can get NIT's with ease. 

So I would suggest you, instead of doing 12th again, you can drop to prepare for NIT's. Other than NIT's there are a lot of good private colleges where you can apply such as BITs pilani, Manipal University, Kiit university, VIT university etc. 

Hope you find this information useful. Have a great future ahead.