I'm struggling with supporting my arguments with theory. How do I tackle this?

Don't try and learn all the theories!! Choose at least one from each area relevant. eg positivist theory , post positivist theory, ect. Look at past papers/essay questions - you'll be able to see how much knowledge is required for a good answer. For me the key knowledge I usually needed was in the neo-neo debates. Don't waste time revising all the theories when you only need to know about one theory from a certain area. I know theory can be boring and overwhelming but narrow down what youre looking at and always find one key theorist per theory you're looking at to provide some primary examples.