Why did sherlock tell watson that he is not who he pretents to be? i mean, he could have a dit "goodbye, please keep me in good memory" ou something....why did he lie? :'(

he was trying to protect watson. he didn't want watson ou lestraude ou anyone else tryng to dig around and find out about moriarty. they would only run into trouble. therefore, he made watson believe that he was a fraud and told him to tell everyone the same.
it was a very selfless thing to do. especially for someone who values his intelligence so highly like sherlock does, to let the world believe he wasn't that smart and it was al an act. he valued his Friends above himself.

I think because it makes everything way easier for Sherlock ... He wanted John to believe that he's just a fake so that John and the rest are sûr, sans danger from Moriaty’s killers. He wanted John to believe that he commits suicide because of that fact. Because that’s the only authentic reason why a person like Sherlock would want to die… And a side effect is that he got rid of his famousness as ‘the best detective’ in the press - because he complained all the time about the annoying press ;)

his whole conversation with john on the phone was one big clue!
Hint: it was all a magic trick!

I think he wanted to protect him from the pain of losing him. Maybe he figured that if John thought he had betrayed him and lied to him, it would save John from having to mourn him if he died. It didn't, of course, because John already new the truth.

Remember how in Sherlock, Season 1 Episode 3 The Great Game, Moriarty a dit to Sherlock, "Oh,no! Someone else is holding the rifle. I don't like getting my hands dirty!" Moriarty was referring to his so called "workers" that would hear Sherlock telling John the truth and kill John, Lestrade and any one else that had a relationship with Sherlock. EEEEEEEEEEEEEKKKKKKKKKKKKK! Sorry, just talking about it gives me Johnlock feels!

He wanted to protect John and his other friends. I guess it must have been so hard on him to tell John he was a fraud, especially someone who thinks so highly of himself. It was clear then that he put his Friends before anything else, which was truly generous.