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!!