And now, I don't thinkfind my English speaking inadequate, as if I have a goodlow score in English speaking.

Do you still have fears afternow you've growingn up?

The original sentence is grammatically correct, but sounds more like a child asking it of an adult, rather than an adult asking it of other adults. Given that in the following sentence you answer in the affirmative, i would suggest the change, which makes the question more suitable for the rhetorical manner in which you're using it.

Some of you might have watched some of the videos I posted on YouTube.

Again, no particular problem with the original, but adding "of the" makes the flow a bit more natural.

I originally wanted to dub them in English, but I didn't dare to.

Actually i'm not sure whether your original sentence is grammatical or not, but i think it sounds better to add "I" and omit "to".

Maybe it's because when I was a kidt school, I had to try my best to achieve a good score at schoolget good grades in English.

If you move "at school" to the start, it implies your being a kid. You could also say "when i was a kid at school". I think it also helps to specify the fact you needed English grades, if that was the case. Using "get" is a little more informal, but using "achieve" sounds kind of stiff.

And now, I don't think I would have a good score in English speaking.

Maybe because when I was a kid, I had to try my best to achieve a good scorEnglish grade at school.

Alternatives:
"I had to try my best to achieve good test scores at school."
"I had to try my best to achieve good grades at school. "

And now, I don't think I would have a good score in English speaking.

DMORE NATURALLY: As an adult, do you still have fears after growing up?

A culturally interesting question. In many cultures, many adults fear death.

ALSO POSSIBLE: I originally wanted to dub them in English, but didn't dare tolacked the courage.

Maybe because when I was a kid, I had to try my best to achieve a good scores at school.

NOTE: The plural is better here.

OPTION A (if referring to general English ability): And now, I don't think I have a good ability to speak English.¶
OPTION B (if referring to a specific English test): And now, I don't think I have a good score in the **** English sSpeaking Test.

One of them is about speaking English.

Some of you might have watched some of the videos I have posted on YouTube.

I originally wanted to dub them in English, but didn't dare to.

Maybe it's because when I was a kid, I had to try my best to achieve a good score atin school.

And now, I don't think I have a good score in'm good at English speaking.

Not gramatically wrong, I understand the intent, but we English speakers won't "grade" you on speaking abilities with a point system. This sounds more natural to say.

One of them is (about) speaking English.

Without “about” sounds more natural. I’d generally say “my fear is…” and “I worry about…”

Some of you might have watched some of the videos I posted on YouTube.

I originally wanted to dub them in English, but didn't dare (to).

Maybe it’s because when I was a kid, I had to try my best to achieve a good score at school. / good grades in school.

In US English we would typically say “grades” when speaking about general performance or specific assignments.

AndBut now, I don't think I have a good score in English speaking.

“And now” works better when something in the present is the same as it was in the past. Eg “I was good at soccer as a child. And now I still play it on the weekend.” Or “As a kid I hated broccoli. And now, I don’t have to eat it!” In either direction (positive / negative) the before and after share the same sentiment.

You could say “But now…” here. In the past you wanted good grades (a positive thing” BUT NOW you think you don’t have good English speaking skills (a bad thing in this context).

Alternatively, you could use another time indicator like “currently” etc.

I originally wanted to dub them in English but I didn't dare to.

And now, I don't think I have a good score inam good at English speaking.