hi,

see you have already taken a year gap and appeared in improvement exam, right now it would not wise to take again gap , however you may join national Institute of open schooling to enroll in 12th course again to secure your expected score to become eligible for nits and iits. nios certificate is valid for most of the institute in India , I suggest you to go through the official website of NIOS and also contact with the nearest NIOS center.

https://nios.ac.in/admission.aspx