Mental health. Seems impossible.

I'm a final year student pursuing my bachelor's of technology degree in Engineering Physics at Delhi Technological University. I've always wanted to contribute to the science community and pursue research as my career. And now, when the time has finally come, I feel as if I don't know physics anymore. I can solve any problem pertaining to mathematical and physical sciences but I don't really know what it is that I end up feeling like I don't know these sciences the way I used to know before. As a matter of fact, I have always been among the top students when it comes to physics; given my Physics score for JEE Advance was 115/120. Also, I have done well in all my exams but it doesn't feel the same anymore. And not knowing what the reason is, makes it all the more disturbing and annoying. And the consequences are all extremely negative as I am unable to focus on my health, and academics. In short my efficiency and productivity has hit the lowest of levels & a fightback seems impossible.