The difficulty of learnning English ?

I think the most difficulty thing in about learning English is that lack of env to practice it. We remember a word, but when we don't use it for a long time, then we forget it again.

I am not sure what you meant by "env", so here are some words that can be used in place of env: 

lack of anyone to practice it with

lack of situations to practice speaking it

I hope this helps :)

go go go

I do not know what to say at the moment, as I am feeling a little confused with my life. When people start to do something new, they are always energetic. *but As time passes, however, they will always feel exhausted. I do not know the reason, maybe it is just because the the human beings are not robots.
We can not do things as what we have planned without any change. (Or you can say, "Things don't always go as planned." or "Things don't always go according to plan."). Life is complicated, which and we can not will never understand it fully for ever. *What We can, however, simply enjoy living. do is just feel and enjoy it. There are some great (many) people who can control their whole life, but just a small part. not their whole life.

Things in blue are suggestions. I crossed out but and what, because usually, they are not good to start sentences with. However, we do use them in speech. I just wouldn't recommend starting written sentences with these words. Also, by saying "some great people" in your last sentence, did you have someone in mind? Or did you mean quantity? I put "not their whole life" in orange because you do not really need to put it in your last sentence. It is a slightly repetitive since the reader already knows that we are talking about their life, so I added "whole" to the beginning part of your sentence.

I really hope I didn't just confuse you! I am trying to learn to become a better teacher >.<! If I confused you, please forgive me!!