Do you feel you aren't making any progress?
As you learn more and more, your progress will seem to slow down. Don't worry, you are still learning, it just takes longer. Look at some past exercises or written texts, see if you can do better. You may be pleasantly surprised.
Are you still making the same kind of mistakes?
Try to spot any patterns.
Be on the lookout for spelling and grammar mistakes that crop up again and again. Find exercises on the Internet that test those particular points.
If you are enrolled in an English class ask your teacher for feedback.
I know I often say you learn from your mistakes, but if you keep making the same ones then they're just bad habits.