Dear_____________,
Thank you very much for_____________.To be honest,__________.However,the current situation does not allow me to do this.
This has been a difficule decision for me,but I have to do this.On the one hand,____________.On the other hand,__________.In view of these,I regret that_____________.I hope this does not bring you much inconwenience.
I feel very sorry to disappoint you.If___________,I will __________.I do appreciate your________.Meanwhile,I wish___________.
Sincerely yours
___________