I came from a literature and history background in high school, and now at university i'm doing a double major in german and applied linguistics.
Basically, i find that most people are really quite interested in applied linguistics when they start learning, as its so damn interesting! what most people seem to find difficult is the heavily applied stuff, such as phonetics, syntax and morphology.
Phonetics at most universities is a HARDCORE course, even at the undergrad level, as there's so much theory, but you have to combine it with so much practical knowledge. For example, the first things we learnt about were the places in the vocal tract, then we learnt about what sounds are produced in the various places, and the constraints and variations that can be placed on these sounds. Then moving on to acoustic phonetics, then back on to practical.
In order to do well at many of these subjects within the applied linguistics field, you need to be willing to put in many hours of not just class and reading and research, but also simply honing your skills. You will find yourself second guessing much of the work you do, as much of it is highly subjective, yet there is still arguable a right/wrong answer.
In short, you should try and get some background info before you throw yourself into a discipline such as applied linguistics. It's not hard, per se, but requires many man hours before you start being GOOD at the discipline.
Good luck!