Like you said, it's a subject that I reckon gets exponentially harder (and more time-consuming) to study for as your study score aim increases. Well all subjects are actually like this, but biology even more so because there are very few sources which fully explain any concept, and it takes quite alot of googling and multiple textbook scavenging (I had to go through my brother's university bio textbooks at times) to find what you need sometimes. So it really depends how good and how fast you can understand things, because at a ~40 level, you don't need to understand COMPLETELY everything; just an overview of everything I guess. So yeh, you could continue, but it's quite likely choosing another subject could yield a similar mark with a much lower stress and work load - that is, unless you reckon your interest in it will motivate you to study?