Or, my biggest problem is that I always makinge the same mistake. Or, my biggest problem is that I always make the same mistake.

US alternative: Or, my biggest problem is that I'm always making the same mistake.

Or, my biggest problem is that I always makinge the same mistake. Or, my biggest problem is that I always make the same mistake.

Again, and again. Again and again.

No comma is needed here.

We can’t change what already happened already. We can’t change what already happened.

US alternative and this sounds more natural.

We can’t change what happeneds already happened. We can’t change what has already happened.

Or “We can’t change what happened.” “Has” 帶有／強調「已經」的含意，所以可以配上 “already” 用，我個人認為比較自然 :)

In my case, I will make the same mistake a third or even a fourth time. In my case, I will make the same mistake a third or even a fourth time.

or In my case, I will make the same mistake three or even four times.

In my case, I willould make the same mistake thirdree, or even fourth time.s. In my case, I would make the same mistake three, or even four times.

The taste of success becomes a “once in youra lifetime” thing”. The taste of success becomes a “once in a lifetime” thing.

I'm not sure what you're trying to say here so this is my best guess.

The taste of success becomes a “once in youra lifetime” thing”. The taste of success becomes a “once in a lifetime” thing.

你想表達的是「一生一次」的意思嗎？

We all keep trying, keep failing, till one day “success” will knocks on your doors. We all keep trying, keep failing, till one day “success” knocks on our doors.

Life is nothingn't fair but that is life. Life isn't fair but that is life.

This sounds more natural in the US.

Life is nothing fair, but that is life. Life is not fair, but that is life.

Do work hard to find your fairgood days. Do work hard to find your good days.

This sounds more natural in the US.