It seems that you are using good prep books at least
MGMAT should brush up your GMAT math. Based on what you said, its still hard to give you concrete advise I am afraid, although I would strongly recommend you to take a mock test! if you don't want to waste GMAT prep test for now, do one of your MGMAT Cat, it is widely accepted as relatively accurate Cat, maybe slightly difficult than GMAT prep but should be comparable with actual GMAT. MGMAT will not only measure your performance/score, but also provide you really cool tools to analyze your performance! So do take advantage of that! You will know your strengths and weaknesses after that! Hope it helps!
And dont be scared!
The more you practice, the easier it gets!
Don't forget Kudo if you like my answer