Here is a revised version for you to compare with yours.
" Dear Mr. Jackson
I am very sorry that there was a confusion about the date of our meeting.
When I heard that you had come to my office to see me, I was so surprised since I had the appointment written down for Thursday and my secretary is currently away, so I could not check with her. I confirmed the appointment with your secretary for Thursday 10:30 in my office. I believe that your secretary may have intended to say Tuesday for the meeting but due to a slip of the tongue, she said Thursday, which was fine for me.
I am sorry that I was not there and that the meeting did not take place, since it was quite important for both of us. I would like to make a new date for this meeting as soon as possible. Please call me to arrange for an alternative time and date.
I look forward to hearing from you in the near future.
You must remember to start each sentence with a capital letter.
You must always use a capital "I" for the personal pronoun.
Proper names [such as the days of the week and personal names] start with a capital letter.
Student or Learner