'I don't study biology at a-level so I feel like I can't do it, especially because most of the people in my class do and I can't keep up'

Biopsychology and Neurology are actually a lot easier than they initially sound. I do not think it is necessary to study biology. It might help, but it certainly does not need to hold you back. It's understandable to feel overwhelmed when your classmates seem to grasp something quicker than you do but they are bound to pick the concepts up quickly if they have already learned them before so this is completely normal!
When it comes to biopsychology and neurology tackle one thing at a time, and in your own time. For example, it is very difficult to understand why your brain communicates with other parts of your body, such as your glands, without first understanding how neurons and synaptic transmission work. The key to writing a good answer with both AO1 and AO2 in it is understanding what you are talking about. If you just try to memorise you are unlikely to be able to effectively evaluate and will therefore drop a few marks, plus it's a lot easier to forget. It may take time but eventually it will click and when it does your evaluation marks in particular are likely to rise.