A language is like a tool. It will get rusty if it is not used for a long time. Your English is quite good, but in order to keep it at top level, you will have to continue to practice reading, writing, and speaking it on a regular basis. Your feeling is an accurate assessment of the situation, not a disorder. We have a saying in the United States: "Use it or lose it." Perhaps you can find an evening or weekend refresher course to take, or there may be an English club in your area where people get together to practice their oral language. Once a brilliant student, always a brilliant student, if you keep studying.
As for your paragraph, there are a few small mistakes, which I have corrected in capital letters. Brackets mean a word should be omitted.
It's been A long time since I [have] stopped studying because I have already completed my post-GRADUATE EDUCATION AND I am working now. [and] I had been a brilliant English student in school and also in college. I have taken THE IELTS twice and I [have] SCORED 8.0 and 8.5 respectively. Ever since I [have] stopped studying, I have started feeling that I have forgotten THE English language. Every time I write or SAY anything in English, I feel that I am not writing or speaking correctly with respect to grammatical accuracy. It has been quite some time THAT I HAVE BEEN away from regular studies, and I just wonder why I am getting such feelings. Is it something natural? Does it suggest any disorder/syndrome, etc.? What should I do ? Please help me and reply properly with valid reasons that support YOUR justification.and also tell me If i HAVE made any grammatical errorS in this whole paragraph.