One possible version:

Dear Jack,

   I'm very glad to receive your email and very delighted to know that you're getting along well with your

Chinese learning. You must be proud of yourself for the progress your have made, so am I.  

   Yes, it is true that the more you learn Chinese, the more questions you will have about it. Idioms are

always a difficult part of a language. "should be one of the difficult ones. It means when we meet a

stranger and become friends, we hate it that we should meet each other so late. In other words, it would

have been better if we had met earlier.

   I hope my explanation can help. And best luck with your learning.

   Best wishes

                                                                                                          Yours sincerely,      

                                                                                                                  Li Hua