If I had a dollar for every student that said they are a "bad test taker," I would have a gorgeous mansion overlooking the beach! Many students use the bad test taker line as a crutch which holds them back from achieving their goals. Instead of viewing it as a skill to improve, they see it as an aspect of their personality, inflexible and unchanging.
If you want to improve your test taking skills, there is one simple concept you must understand. Tests are NOT special. They do not deserve special treatment and they will not make or break you. In fact, tests are an exciting opportunity to showcase your knowledge that you've worked hard to acquire. Do not put tests up on a mountain and look up at it as if it is unobtainable.
A large number of students make test days stand out in their life. They eat differently, go to sleep at different times, change their study habits completely, and show up on test days out of sorts. Of course they will be nervous, because everything about that day has been made different! You have alerted your body to the fact that today is important, which triggers your nerves and anxiety (all which hurt your overall test scores).
The fix: treat every academic day the same. Create good habits that can be consistent on "regular" days and test days. Study consistently instead of cramming. Eat the same types of breakfasts and go to sleep at the same time each night. Your body won't understand that the test day is different, so your nervous responses won't trigger. Imagine how well you will perform with no pressure and added anxiety!
There are many ways to improve your test taking skills, but start with the easiest and most important solution - consistency. Do not allow yourself the chance to make the test stand out or feel special. Treat it the same way you treat every day, and watch your performance be as strong as it is each day in class.