Thanks for writing to us.
It seems that you are doing something which was not your first choice, but something which you choose out of peer pressure or societal pressure. YOU ARE NOT ALONE in our society who is facing such an issue. Today most our young Indians go through this. B, tech BE courses has become like their safe zone/ comfort zone and they are afraid to experiment. You have mentioned that you scored 9.8 grades in 10th and later choose science stream since then you have noticed a decline in grades and interest in life. This is happening because you are not doing what you are good at or what you always wanted to do. It looks like you are very judgmental about yourself, stop being that. If you judge fish by its ability to climb a tree then you keep thinking fish as a stupid entire your life, if you measure success in terms of academic grades then Einstein would be the biggest failure in the history.
Think about what you want in life, Do you want things which are accepted by rest all in society or you want things which give you satisfaction and joy?
Now let us come talk about what you can do to feel better
firstly; have an open conversation with your parents, as you have mentioned that your parents are very understanding so they will definitely understand what you are going through if you confront your feelings and tell them what is happening with you. Don't safe alone, seek support.
Second; start working on your passion, irrespective of anybody accept it or not. You need not discontinue your course, but along with that start writing what you like. It might not be a published book at least start with something for your satisfaction
Third; last but not least, be mindful about your thoughts and feelings. See how you are feeling in most of the day, if you have any self harm thoughts or feel low most of the time, then talk to a professional seek help.
wish you good luck in life.