Hi Faizan!
You have mentioned in your question that your results in the 11th standard were bad. But I think that if you put in all your efforts and prepare well for your 12th standard exams, there are chances for you to cover up. In that case, you may have the chances to reach the criteria required to clear JEE. Meanwhile, you also need to prepare for JEE simultaneously so that you do not mess up in this exam. 
But at the end of the day, if you manage to cover up your intermediate marks, there are high chances of you clearing other entrance exams and get a good rank if JEE turns out to be unsuccessful. So, I suggest that you focus on all possible entrance exams other than JEE also so that you have multiple options with you by the time you finish your intermediate. 

Please let me know if this answer was helpful to you and all the best for your future!

Off course, dear Faizan, everything is possible in this world with hard-working, dedication and patience. I suggest you forget about the past just make a new start with lots of positive energy just focus on your work in the right direction rather than think about your past or doubting your self. I totally believe we can do anything with a focused and dedicated mind.

All the best dear

Hello aspirant. Yes you can definitely get a good score if you work hard on. Don’t look at your past performance. Focus on your present and future. Make daily schedule of studying in your class 12th and give appropriate time to your studies. You will definitely achieve good marks. Considering JEE Mains. It is a highly competitive exam so start preparing as soon as possible to get a chance to clear the exam. At the end, don’t lose hope and try to improve yourself day by day. I hope you found this answer helpful. Good luck.