Excuse me, but I do not believe this concerns you. I beg your pardon, but this is a private matter. I am really sorry, but we wish to have this conversation alone (or, you could replace "alone" with "in private"). There are many other ways to say this. Unfortunately, to many people, being told something is not their business, no matter how polite you are, will seem rude to them. That is their problem, not yours.

what happen，，say with me,try my best to let U smile again

Allen has all the good replies laid out for you. I try to use as few words as possible. The phrase, "it's personal" fits many situations and usually is not interpreted as being rude. There are simply many matters in life that are...personal.

in my opinion i think it should be "can i solve this problem by myself?"

"It is personal." or "I am sorry, but I cannot tell you."