Recently, the weather has been becoming even more scorching!,

"Scorching" is a really strong modifier to mean hot, so including "even" here for emphasis fits well

Never had Ilike I have never seen!

This line can't stand on its own as a sentence, but works well if you combine it with the previous one

Do you have any tips for cooling the bodydown?

We typically don't mention our bodies directly when talking about being hot or cold. "Cool down" is a regular expression that works well here

Basically, I stay in the inside my room with air conditioning.

I havesweat a lot of sweat, which makes me feel my skin sticky.

For sweat, we'd use it directly as a verb like this rather than as something that we "have."
Like "body" with hot or cold, no need to mention your skin here directly, it's understood in the context of sweating.

I cannot endure it happening.

Let u's bring another interesting thing into this/the diary.

"Let us" is correct, this is just a stylistic edit. "Let us" is almost never used as opposed to the contraction "let's" because it sounds very old-fashioned/outdated

Suddenly, I felt some liquid hit my arm, and I just cayelled out "shit" spontaneousautomatically!

"Call out" would be more for getting someone's attention: "He called out to her, but she didn't hear him." But you can yell/shout for any reason.

"Spontaneous" tends to mean without any reason/motivation. Here, you had a reason (you got surprised). Doing something without thinking about it/reflexively can be described as "automatic"

So I looked at my arm, and I(I) realized it's a re was actual bird dungpoop!

You don't have to repeat "I", you can borrow the first one. This is part of a story => past tense "was"

"Dung" is more reserved for scientific or technical contexts for animals. As part of a casual story, I'd just say poop. If you want less cute/more vulgar, "crap," or more vulgar still, "shit"

cCrap!

Can you imagine how stinky it iwas?

But, in China, if someone had beengets hit by bird dung, it means you are lucky!

This is a generality, no need to speak in the past tense here

WhyHow could I have forgotten to buying a lottery ticket yesterday?

You can't say "why could," "how could" is set together. However, "why" does work at the negative. E.g., "Why couldn't I have remembered to buy a lottery ticket?"

Now, I feel like I lost onea million RMB.

More natural to say "a" instead of "one" here

CI'm crying...