The l(A) Lesson is lLearned

Todayhis morning I had an interesting but also a strange experience.

I had an interview with 3 respresentatives of a big and famous company.

During the interview, I suddenly forgot all themy words and ended up sitting silently.

It was frustrating and (made me feel unprepared / left me feeling unprepared).

The phrase "left me feeling unprepared" is a little more idiomatic.

It wasThere were already two or three times when I was speaking in Italian or German because I know those languages; rather bad, especially for an important business talk.

The semicolon separates the comment on speaking in Italian and German - without the semicolon is sounds like "rather bad" is describing your Italian and Germany skills.

I feel frustrated now because I wanted to show my best myself but in reality I showed that I unwasn't ideal and "lost my face " in front of others.

"my best self" already includes "my".

I think it happened because I was too nervous and overthinking things.

Next time, I will try to stay calm by taking deep breaths.

The lLesson is lLearned

Today / This morning I had an interesting but also a strange experience.

"Today morning" does not sound correct.

I had an interview with 3three respresentatives of a big and famous company.

During the interview, I suddenly forgot all the words and ended up sitting silently.

"forgot all the words" is unclear.
Which words? Words you prepared, or how to speak at all? It would help the reader to understand if you were more specific.

It was already This has happened two or three times when I was speaking in Italian or German because Imy knowledge of those languages rather bais limited, especially for an important business talk.

I feel frustrated now because I wanted to show my best myself, but in reality I showed that I unidealwas not ideal for the job, and "I lost my face " in front of others.

If you felt embarrassed or humiliated then you did lose face, so no need for quotations around the expression.

The lLesson is learned

"The lesson is learned." <--- okay
"Lesson learned" by itself is a commonly used phrase for when you learn from something bad happening to you

Todayhis morning I had an interesting but also a strange experience.

You're only talking about one experience, so "an" is the article that covers that
With what you had, it almost sounds like you're saying "This morning I had an interesting experience but also a strange experience" (two separate experiences)

I had an interview with 3 respresentatives of a big and famous company.

During the interview, I suddenly forgot all themy words and ended up sitting silently.

Grammatically good, "all the words" sounds a little odd here because when you use "the" it sounds like you're referring to a specific subset of words, such as a speech, or lyrics to a song, not the entire English language. Now, if you had rehearsed a speech before the interview and had mentioned that in the previous sentence, and you were referring to the words in the speech, it would be natural.
"my words" is a sort of set phrase that can refer to your own personal lexicon, "forgot how to use my words," "forgot my words" etc

It wThis has already happened two or three times when I was speaking in Italian or German because I know those languages rather badly, especially for an important business talk.

I feel frustrated now because I wanted to show my best myself but in reality I showed that I unideal'm not an ideal candidate and "lost my face " in front of others.

Edit: I had a brain fart and forgot this was an interview, in this case "not an ideal candidate" sounds more natural
Otherwise it sounds like you're saying you're not an "ideal human being"

Next time, I 'll try to stay calm by taking deep breaths.