I’ve never written this much in English before, butand I’m proud of myself.

I'm excited to keepcontinue learning and improving every day!

Adding some words may make this sound better and add more to the sentence

Until then, I have to study hard, and I believe in myself.

No need to repeat "I" the second time. No comma needed.

I’ve never written this much in English before, but I’m proud of myself for doing so.

Not required. The sentence above was great and understandable. It is just a bit better to clarify the reason you are proud of yourself.

I feel confident about this job because I really want to become a cabinflight attendant.

In American English, at least, we usually say "flight attendant."

I’ve created a fast learning planplan to learn English fast with the help of ChatGPT.

The way you wrote it isn't wrong, but this sounds more natural to me.

I’ve never written this much in English before, butso I’m proud of myself.

I feel confident about this job, because I really want to become a cabin attendant.

This website was recommended to me, and one of my first tasks iwas to introduce myself.

wWrite aAbout yYourself

It's a small thing, but most words in titles are capitalised in English. The exceptions are small words like "a", "an", "of", "the", and so on.

Until then, I have to study hard, andbut I believe in myself.

"And" is not technically incorrect, it just sounds less natural.

I’ve created a fasn efficient learning plan with the help of ChatGPT.

"Fast learning plan" kind of sounds like the plan itself is quick, rather than you learning English being the quick thing.
You could also say "a plan to learn quickly" instead.