Disappoint
Disappointed

All these days, I thought a lot, and went through an indelible thing,
Recently, I've been thinking about an experience I can't forget.

it was so disgusting, gross, freaky that makes me have a bad impression of Londoner,
It was so disgusting, gross, and freaky that I now have a bad impression of Londoners.

I know, I shouldn’t take a part for the whole, but I really can’t help it.
I know I shouldn't generalize, but I really can't help it.

I don’t want to think like that, but the feeling can’t be cheat.
I don’t want to think like that, but I can't deny my feelings. 

I really shocked about that thing. As we all know, first impression is very important.
I was really shocked about that thing.  As we all know, first impressions are very important. 

However, the behaviors can’t be neglect, too.
However, experiences can’t be neglected, either. 

I oughtn’t to generalize about people, absolutely, but insult people are a rude behavior.
I shouldn't generalize about people, absolutely.  Insulting people is rude.  
[Were you insulted or are you planning to insult someone else?]

I don’t approve judge people solely by their appearance, but I want to say some people’s conduct is really let me down,and scared me,
I don’t think you should judge people solely by their appearance (actions?), but I want to say some people’s conduct is really let me down.
appearance or conduct?

I afraid make friends with strangers now.
I am afraid to make friends with strangers now.

I draw a conclusion, not everybody wants to be your friend sincerely.
I now know not everybody sincerely wants to be your friend. 

I hope you're ok.  There are good people in the world, but you need to be careful.