I can enlighten you - Vocabulary.
Far too many teachers push grammar, but the real key to successful language learning is as wide a vocabulary as possible.
A limited personal Vocabulary means you are very restricted in the sentences you can create, and also how much you can understand.
The only common factor with weak English language learners is the lack of words at their disposal.
Memorising words is going to be very helpful, but you must use those words in sentences you make up.
One possibility is to examine the conversations you hold in your native language, then work out how to say the same in English, but worry about grammar as a secondary point.
Read, read, read, then watch programs about your favorite subjects, but never bother with Hollywood rubbish.
Reading can be books, magazine pieces about whatever interests you, or news from the UK.