I got something like 17 on my first try because I forgot all my trig functions...this is pretty normal. Once I got down the values of the trig fxns of π/6, π/3, AllStudentsTakeCalculus, and all that stuff, I went up to a 20+. I reccomend taking one every couple of days and near test day once a day. After 3-5 tests I've been scoring 21+ so don't be discouraged. Know that most of the time there are shortcuts to the way problems are solved e.g. knowing your common triangles in solving for a much bigger triangle, is important because it can save you lots of time to do the harder permutation and combination questions.
Don't stress, so long as you learn from your mistakes and learn how to approach a problem it will be fine.