I can't understand this tweet

Question

上には上がいるので自分はすごいんですなんてそうそう言っちゃいけないなとつくづく感じるわ……

本気ですごいでしょ？って言っていいのなんて世界で一番の界隈に入った時だけよな〜〜 じゃないと恥をかく

I'd translate it as:

I strongly feel that, because there is always someone better than you, I can't particularly say that I'm amazing

I could only say that I'm amazing when I entered the best neigbourhood in the world. Otherwise, I get embarrassed

I saw 「X界隈」 can mean "person who likes X", but that doesn't seem to fit here. However, "neighbourhood" doesn't, either.

That's my main issue, but I also particularly don't understand this part:

言っちゃいけないなとつくづく感じるわ

Is that 「な」 the one that adds certain emotion at the end of the sentence?

And I don't really understand this part either:

本気ですごいでしょ？って言っていいのなんて世界

Is that 「の」 the one that adds a tone of explanation?

Is 「本気ですごいでしょ？って言っていいの」 qualifying 「世界」 through 「なんて」? If so, how could I translate it? I understand that the structure 「Aで一番B」 means "the most B among A". So that would turn the translation of the second part of the tweet into something like this:

Only when I entered the best neighbourhood in a world where I can easily say that I'm seriously amazing

I'm definitely wrong somewhere, so please help me.

Answer 1

Your translation of the first sentence is almost perfect except that そうそう in this context means something like "easily" or "too often" (see this; this is a negative polarity item, i.e., it's used in combination with ない). な after いけない is this sentence-end particle that "adds emotion".

As internet slang, 界隈 refers to a vague community of people who share the same interest, or simply a formless group of people. So 世界で一番の界隈 is "(around) a top group in the world".

The second sentence is basically a cleft-sentence, but のなんて is used instead of のは. なんて is a topic-marker-like word used to make light of something. The "base" sentence is something like this, which I think is easier to understand:

本気で「すごいでしょ？」って言っていいのは、世界で一番の界隈に入った時だけだ。

≒ 世界で一番の界隈に入った時だけ、本気で「すごいでしょ？」って言っていい。

It is only when you have entered a top group in the world that you can say "Aren't I amazing?" seriously.