The better you understand the basics, the easier it is to understand the rest, and the better your prior knowledge, the easier it will be to learn new facts. For example, a good knowledge of the mechanisms by which insulin controls blood glucose, makes learning about diabetes easier.
You need to identify your areas of weakness and work on improving them. It can be tempting to spend many hours revising the material you already know because it's easier and better for your confidence - but it's also a waste of time: examiners are very good at finding out candidates weaknesses. Focus instead on understanding the concepts you struggle with and take the time to understand them, and always ask for help if you need it.
1. Think about which subject or topic you find the hardest to understand.
2. Put some time aside to go 'back to basics' for this topic. Find a well-written, easy to understand text book, and start from first principles.
3. Make notes on the basics of this topic, making sure you understand everything you write down.
4. If, at any stage, there's anything you don't understand, go and get help. Ask a colleagues, or a tutor to help you.
5. Repeat this process for all the topics you struggle with.