The primary cause of that was the feedback on my pronunciation, which I recieived from thea native speaker of EnglishEnglish speaker.

Since I usually communicate with non-native speakers, it is pretty challenging for me to get a constructive feedback on my pronunciation.

Whenever I ask people to guess my descent, they never guess correctly.

Most of them say that I am from South Asia; some even assume the Middle East.

Therefore, I unintentionally created myself the illusionan illusion for myself of my accent being adequate enough to not work on it at all.

My personal preference: "created an illusion that my accent is already adequate enough and that I don't have to work on it at all" or "created an illusion of my accent being too adequate to work on"

However, a few days ago, I made an acquaintance with a person from the UK.

Better would be I made an acquaintance from the UK

Later, he told me that the decisiveding factor for him was my incorrect pronunciation of the sound "th''.

I fiound it quite embarrassing, because I remember learning about it in the secondary school.

After this reality check hasd occurred, I decided to approach the phonetics with seriousness.

Theis was primarily cause of that was thd by some feedback on my pronunciation, which that I recieived from thea native speaker of English.

"The primary cause of that was..." is correct but very formal, like you are presenting scientific research or something.

Since I usually communicate with non-native speakers, it is pretty challenging for me to get a constructive feedback on my pronunciation.

Whenever I ask people to guess my descentbackground, they never guess correctly.

You could say "background" for where you came from, "ethnicity" for your race or genetic origin, or just "native language" if you are talking about language. You could say "They never guess correctly" or "they never get it right".

Most of them say that I am from South Asia; some even assume I'm from the Middle East.

Therefore, I unintentionally created for myself the illusion ofthat my accent beingis adequate enough to not work on it at all.

However, a few days ago, I made an acquaintance with a person from the UK.

You could also say "I met a person from the UK" or "I acquainted myself with a person from the UK" or "I made an acquaintance from the UK."

Without any doubt, he has burst my bubble!

"Without a doubt" or "Beyond all doubt" or "Beyond a shadow of a doubt" for strong emphasis.

It turned out that I was substituting the voiced sound with thea "z", and the voiceless sound with an ''f'.

"a" and "an" optional

I find it quite embarrassing, because I remember learning about it in the secondary school.

After this reality check has occurred, I decided to approach the phonetics with more seriousness.

or "more seriously"

Recently, I gotbecame interested in working on my English pronunciation and accent.

'became' is better for formal English. 'got' is quite casual / slangy.

The primary cause of that was the feedback on my pronunciation, which I recieived from thea native speaker of English.

Introducing the native speaker so indefinite article.

Since I usually communicate with non-native speakers, it is pretty challenging for me to get a constructive feedback on my pronunciation.

'feedback' is not countable so no indefinite article.

Whenever I ask people to guess my descent, they are never guesscorrect.

Most of them say that I am from South Asia; some even assume the Middle East.

'the Middle East'

Without any doubt, he has burst my bubble!.

Don't go too made on the exclamation marks.

It turned out that I was substituting the voiced sound with the "z", and the voiceless sound with ''f'".

I find it quite embarrassing, because I remember learning about it in the secondary school.

After this reality check has occurred, I decided to approach the phonetics withmore seriousnessly.

The primary cause of that was the feedback on my pronunciation, which I recieived from theEnglish native speaker of Englishs.

Since I usually communicate with non-native speakers, it is pretty challenging for me to get a constructive feedback on my pronunciation.

Therefore, I unintentionally created for myself the illusion of my accent being adequate enough to not work on it at all.

Without any doubt, he hasd burst my bubble!

I find it quite embarrassing, because I remember learning about it in the secondary school.

There's nothing to be embarrassed about.

After this reality check hasd occurred, I decided to approach the phonetics with seriousness.