I think that you feel nervous because you are not sure about your ability, about the chance to get high score. This means that you need to practice more and more.
What materials have you used? If you are stuck at understanding verbal concepts, try
Manhattan GMAT guide set, or try online courses: e-gmat, Veritas,
Magoosh,...
I suggest that you register a free account of Veritas website, and practice free questions from their question bank. Their question bank contains a lot of questions, especially SC questions. You could take many as you want and remember every rule/concept.
RC/CR lack of vocabulary? There are no other ways but try to read as much as you can to build up your vocabulary.
It is said that GMAT doesn't test vocabulary but logic skill. However, in order to earn high score, we need to have a wide vocabulary.
It is also said that in some GMAT questions, even if we don't understand some words in those questions, we could still guess based on the context. This trick doesn't help much, however, if we dont have a wide vocabulary. Thus, trying to build up your vocabulary is the only way to solve your problem.
TIME: No one could help you arrange your time. You must do it by yourself.