I understand where you are coming from my background is very similar to yours in the sense of having an excellent teacher which you excel under.
You say you have been doing well in your homework but getting the occasional D/E/U in tests. The 2 things that spring to my mind are either you're Struggling under time constraints in class for the test or you're panicking once you getting into the test. If it's time constraints then this will just get better with more and more practise.
If you're really willing to put the effort in here's what I would personally advise. Use Examsolutions (It's brilliant) revise any topics you need and once you feel you have enough for the module do a full paper and see what you can get. He has full papers with video's on how to do each question so once you have completed the paper you can watch video's on any question you struggled with.
If you're still struggling then post any questions on here or you could even message me personally I'd be happy to try help you as long as it's not M1
But I guess it also depends on what grade
you specifically want at the end of the year