Don’t make athe same mistake twice Don’t make the same mistake twice

In my life, many “big brother”s” have told me the above statement many times. In my life, many “big brothers” have told me the above statement many times.

(1) "Mentor" might be better to use here than "big brother" and (2) I think "have told me" is more natural (i.e. it happened at unspecified times in the past like yesterday, and last year, and the year before) vs. "told me" (i.e. it happened at some specified time like yesterday)

But you know what happens? But you know what happens?

I always forgoet and keep making athe same mistake twice. I always forget and keep making the same mistake twice.

However, this time I remembered. However, this time I remembered.

Changing to past tense for consistency throughout the passage

Last month I tryied to buy a pc joystick from the internet. Last month I tried to buy a pc joystick from the internet.

Because you said "last month," "try" should be in the past tense (plus the whole passage has been in the past tense)

I asked a few questions to the seller and s/he answersed me almost immediately. I asked a few questions to the seller and s/he answered me almost immediately.

So, I believed I willould be making a deal with him/her. So, I believed I would be making a deal with him/her.

When we set up the meeting pointappointment, he didn't show up and I stood and waited in the train station for an hour. When we set up the appointment, he didn't show up and I stood and waited in the train station for an hour.

Because this starts with "when", this is a subordinate clause and is incomplete without a main clause. You should add a comma to the end of the sentence and add on the next sentence

He didn’t show up and I stanood and waited in athe train station for an hour. He didn’t show up and I stood and waited in the train station for an hour.

Using "a" before train station sounds like you weren't sure which one you were in (i.e. you were kidnapped or disoriented), which I don't think is the case here.

If I asked for his/her contact number that will neouldn't haver happened. If I asked for his/her contact number that wouldn't have happened.

I'm not sure I understand what you wanted to say here, but I think it might've been this

Tomorrow I will try to buy the same item again. Tomorrow I will try to buy the same item again.

I think you need to add "try" here, because the first time failed, right? Otherwise it sounds like you were successful the first time, and will be buying a second item.

This time I must ask for the seller’s phone number. This time I must ask for the seller’s phone number.