The most difficult thing I think is being scared to make mistakes. English is an easy language to speak badly.

I encounter difficulties in the nasals, accent, grammar, proper pronunciation, spelling, subject-verb agreements, proper para-linguistics and extra-linguistics, proper tenses and word formations.

My students tell me it's plurals and tenses.

I have to agree with Kevin. He's totally right! A lot of people are being scared of making any mistakes, speaking errors, writing errors, doesn't matter they are still afraid. If you ever want to learn some language as well, you have to forget everything about your native lingo and comparing nothing to it. The English grammar is easy to be learned as well. Just keep going and you will see how well you will progress! :) Wish you luck!

the most difficult thing would be writing in English even when you are advanced, but it is not impossible to make mistake in spelling very simple words. Now I can listen and read high academic contexts but I always make mistake when I am writing or speaking! and I don't know why and how can i avoid it! good luck