It's onlyjust after 4 am now, as always, I woke up againwhich is when I usually get up anyway.

I think this structure sounds better if you want to draw attention to the fact you wake up around 4am.

I saiAs mentioned before, I always study English when I 'm awake up at nightat some unearthly hour.

"As mentioned before" can make your text sound more diverse, since you know it's you who is studying, you can however still add "As (I've) mentioned before", we have this expression also "some unearthly hour" generally refers to the middle of the night, which you could've also used "when I get up in the middle of the night".

Today I am going to talk about a little feeling from mymy feelings towards learning.

"my feelings towards" is a common expression, you might often see this, used to indicate your general opinion or thoughts you have on a particular topic.

Learning English is really hard and it takes quiterather a lot of time.

Ok, so while this sentence is "essentially perfect" I think the word "rather" sounds more appropriate to convey the immense amount of effort required, I also removed the "it" to make the sentence flow more naturally.

There are too many knowledge pointdifferent aspects, like idiosyncrasies and irregularities to learn and remember.

"knowledge points" sounds too unwieldy, I'm sorry, we don't really use such an expression, although you can say "bullet points" to refer to a list of items or other things.

For me,I find that every day, I will meet someencounter new words, new usage or new slang which I'm not yet familiar with.

It's not necessary to put the word "new" before all three (however maybe it might be - I even remember watching youtube videos about lawsuits which have involved petty things such as the positioning of commas on legal documents, therefore I'm not entitled to say you were wrong, however I think just USUALLY we don't repeat adjectives in such lists. "For me" at the start of a sentence sounds too unnatural also I think.

I once thought I could master English within maybe three years, but I realize that it's totally wrong nownow, this expectation is completely unrealistic.

First part of the sentence before the comma - perfect, for the last part I tried to paraphrase it in a way which I thought reflected your thoughts, if I was wrong then I apologise. We do often use these words "completely unrealistic" together, or you could say "but I realise now, how utterly wrong I was." as an alternative.

Learning English is a long journey you have to go onendure.

you could just put simply "you go on" without the "have" but since you put "have" I thought it was better to change it to endure, I hope this is ok, I'm just trying to provide different alternatives.

Maybe it will take you several years, decades or even a whole lifetime.

Absolutely perfect structure - just needed tweaking slightly by changing "life" to "lifetime".