Dear Sir or Madam,
This is ----(你的名字). The purpose for me to write this letter is to express my genuine appreciation. This would be an unforgettable experience throughout my life.
Last time, I met some difficulties in ----(遇到什么困難)，and you provide me ---(提供了什么幫助) after you heard my sufferings without hesitation. Without you, I could not deal with the problems successfully. Your generous support means lot for me.
Once again, I am extremely appreciated that everything you helped me with my this sufferings. Please let know once you need any assistance from me, I hope I could have opportunity to return your help.