English is frustrating me

I know that I can speak and write in English because since when you're in kinder, English is one of the main subjects and you have this subject every year until your college days. But why do I find English so hard? Maybe I'm just not focused enough when I study. I know I need to study again and again because I dont want people or friends to tease me. At this age I'm not good at speaking in english. I want my grammar to be perfect. Some of my friends are working in a Call centers beacause they are good at english - but me? I didn't pass the interview when I applied for a job at a BPO Company, they always say that I need to improve my Grammar!