I had to rearrange a lot of this. There were several redundancies. And some sentences that I did not understand, such as [I brought his workers and visited Si-heong city to get a education from the store in outlet.] 

Yesterday, I saw my friend. (it's not really necessary to say how long you've known a friend unless it's relevant to what you are saying)

He had a career in sales for a chemistry company but he decided to quit his job and open his own shop in September.

I visitied his store in Si-heong city to learn about his new career in furniture. 

I asked him why he changed his career. He said his major is trade but it was difficult to sell products that he didn't know well enough. 

This has bothered him for a long time. So he tried to work for a furniture company and succeeded. 

I asked him why you changed your job field to furniture.
He said his major is trade.
He felt it's hard to sell products to the customer because he didn't know about product well.
This reason bothered him long time.

And he try to go to furniture company.
Then he succeed in furniture field.