I think you should study hard you will better, there isn't any other way

First of all don't beat yourself up about it.. grammar is important even with a lot of mistakes you will still be understood. However I do understand that you want to get better in it. You have to focus on certain topics and practice them A LOT, then move onto another topic that builds off of that one so you practice both at the same time.

Try to define the areas that you need to improve: select the topics and focus on each of them for a few days. If you still feel that you haven't improved enough, you can contact a teacher on italki and have grammar lessons where you can practise it. Good luck!

You have to read, my friend. English is a very confusing language and is mastered by sheer repetition. Read short stories, novels, anything and everything. It will click in time (make sense in time).