There are many reasons to that. 1. You didn't study hard enough. 2. The way you studied was not active learning. 3. You didn't get the proper guides to learn. Do you learn English out of your own interest or do you learn English because you have to? Both answers will each lead to a different result. If you learn English out of interest, I will be surprised that your English doesn't improve much after years of working hard. Is it because you set your goal too high that even you make small progresses, you are not satisfied by them? Learning isn't an overnight process. It's a life long one. Especially when you are talking about learning something which is complex such as learning a language. Also remember, it is not how much time we put for studies, it is how much we put into those time when we study. Anyway, you shouldn't give up. You can do it!!

I always tell this: 'Let's see, you say you have been studying English for three or four years and you feel you don't know anything.' Ok. Answer this question honestly; how many hours have you studied per day? Hmm, not much eh? So it isn't four years not three either. Now those hours you did it, how much were you concentrated ? Have you enjoyed while learning it? Do you like English? Runing through the basic, do you really want to improve it? So everything is possibe!. P.S. Don't worry about making mistakes. =)