Why Grammar is so confusing, its hard to remember and if somehow I manage to form a sentences using it, its impossible to speak fluently, how to overcome?
Grammar rules are definitely tricky! One of the biggest reasons that learning and using grammar correctly is so difficult is that there are so many exceptions to every rule. Are you asked to choose between “at” or “in”?
We often say “at home” but then say “in the airport”, even though both are places that you could be. It takes time to learn all these exceptions, and unless you use English (or any other language, for that matter!) enough, it will be almost impossible to be able to learn and master all these different exceptions.
The best thing to do, in my opinion, is practice a lot with a native speaker. You can ask a teacher to correct every mistake that you make. If you write them down and keep track of them, you can understand the mistakes you make and improve over time.
If you do not have access to a native speaker or teacher, the best thing you can do is read a lot and watch movies or TV show episodes with captions. Pay attention to different collocations and especially to phrasal verbs and verbs with prepositions, because those are typically the hardest to remember. If you study the verbs as phrases rather than single words, you will be able to remember and use them much more efficiently.
Hope this helps!