Decline job promotion

I have prepared a letter to cease the acting of a higher rank post...I would like to seek your comments and advice on my presentation and grammar. Thanks!

....I am sincerely thankful that I was offered a chance to take up the acting appointment and the work experience during this period is rewarding. However, I have to make a difficult decision to cease the acting appointment due to the deteriorating health condition of my aged mother. Taking care of an elderly parent is not an easy task especially my mother who relies heavily on me for emotional support.

For my next posting, may I enlist your assistance in arranging a post to me which is close to my home so that I can spent more time in taking care of my mother.

I once again thank you for the offer and the posting arrangement.