I noticed a sudden drop in grades and motivation, even with good time management and productivity. Any advice to combat this?

I have been averaging excellent grades (A-) and was able to end my first semester of college with 4.0 GPA. I started 2nd semester with 3.9 GPA. I was averaging 15-20 hours of study a week and did fairly well on my exams. However, when I took all my exams during this current Spring semester, I did poorly, especially in one of the classes that I spent a lot of time in. <br><br>My other grades also started to drop. I'm now managing an average of 12-14 hours of studying a week, and I usually end up sleeping, journaling, or stretching. I have to force myself to study now. My time management seems to be good, and my productivity levels are fine, but I still feel mentally fatigued. I find it hard to focus on studying, and I always feel anxious while studying. <br><br>I've tried everything to combat this, from using productivity tools, to being more efficient, changing my environment, taking breaks, etc. Please give advice on how to combat this.