What can a student who works hard,is motivated to learn do to improve his grades?
I think your difficulty is with prioritizing tasks and committing to them. Don't worry, it is not uncommon, and its easy to solve once you accept it.
First, you need to decide which is more important to you at present, doing well in the exam or learning the subject well.
(Obviously learning the subject well is very important, but the exam grades may be a more immediate and pressing concern. Possibly they impact future prospects significantly, or maybe they are not so relevant). If the exam is important now, then be strategic about it. The deep appreciation of the subject should have started earlier, and if it didn't, you should revisit it once you get through the exam. If the exam isn't so important, then relax and keep enjoying the subject.
Second, about topics to study. A few days before the exam is not the time to play favorites with topics. At that point it shouldn't matter what you like or don't like (again, this is something for either before or after exam week). So spend an appropriate amount of time on each topic. A lot of students spend more time on topics that are graded heavier - this I personally detest, but it is strategic and meets a short-term goal.
Finally, mnemonics and active recall (I'm not familiar but this sounds a bit of voodoo) and the like are probably best left behind in school. I know it takes some pain to change mindset and the way you approach subject matter, but there is a reason university is distinct from school. Sure, use mnemonics when they are a standard tool (colors on a resistor for instance), but don't use them to start committing regular subject content to memory. Even if you have understood the topic well, this will serve as an unhealthy crutch that you'll keep falling back on, and will feel helpless without.
Apologies for being a bit blunt, but one of the distinguishing skills of a university education is realizing that we are responsible for our condition and our actions alone can improve them. Its certainly not late to start seeing that.