I came into VCD out of interest and had no idea about conventions, drawing systems, design processes and so forth. And actually, I used to Photoshop a lot. I was never the strongest drawer, but persisted in trying my best. I can see that you have been practicing so just keep at it and you're bound to improve. I learned that having a fantastic final presentation does not guarantee you success in the subject. The process of improving, learning and manipulating a design you have created into what the client wants is what counts. Sure, you may be the best drawer but if you don't follow this, you won't get anywhere.
Having said that, being a good drawer does help. You are able to (more easily) illustrate what you wish to achieve through your designs. Keep in mind that this is not always the case; you are also able to illustrate your aims through annotating, for example. Being a good drawer is not the only component needed to succeed in VCD.
Half of the exam is based on theory work. Analysing visual communication (images, ads, etc) is a very important aspect in VCD. Understanding professional practice and knowing the design process is just as important. There are always exam questions based on these things at the end of the year. The written part of VCD is generally left out by many student as they might not deem it "as important" than designing. If you feel that you're not strong in the design aspect, make sure you make it up with the theory work.
Just keep practicing your rendering, tonal elements, and your drawing structure. Having an interest in VCD can help you achieve great designs and ultimately, a good study score. Also, create a poster of 3D paraline, 3D Perspectives. List the subheadings (oblique, isometric, planometic, two point perspective, one point perspective, 3rd angle projection) and add examples of each. This really helped me towards exam time.
Reading through the pinned threads might also help you. Feel free to ask if you have any questions (would be glad to help where I can).
Good luck