o think that you need to post your attempts at your letter and then we'll help. Because i think that it's not allowed to give you a written letter, that's what i saw in some previous posts, but just give some helpful sentences or a starting and we will try to help you more
Here is a simple entry of the letter
Dear .........
The purpose of this is to convey to you my sincere apologies for any inconvenience you may have experienced last three years with respect to .........