I think that you need to improve grammar by practicing more and more. Silly mistakes in grammar mean that have not practiced, enough. Practicing grammar means that you are overcoming different types of examples of questions, and that will help you a lot in improving your grammar. But, there are a few more tips, below:-
1. Reading simple story books to famous novels : Rather, than practicing you can also read different story books which will introduce to better grammatical structures of sentences. It will also help you a lot. There are usually two ways of learning - explicit learning and implicit learning. Explicit learning requires conscious thoughts of the learner so that, means that usually the learner is aware that he/she is learning something. Implicit learning is the exact opposite of explicit learning. In implicit learning, the reader does not even know that he/she is learning something. One example, of implicit learning is reading story books or listening to music. At first, start with simple stories like Little Red Riding Hood then, you can proceed your level further to famous novels like "Jane Eyre" and "80 days around the world" . But, remember this a long time process, as you are short of time. So, it will be better that you follow the other tips, for now.